Drop unstable Travis osx in favor of GitHub Actions

For some reason, the Travis osx environment has been really unstable.
It failed on today's cron too:
https://travis-ci.org/ruby/ruby/builds/579843163

As we have almost the same test environment (including OpenSSL version)
in GitHub Actions and it seems to be more stable and faster, I think
there's no motivation to maintain Travis osx CI environment.

By removing this, we'd be able to simplify .travis.yml as well.

.travis.yml

@@ -38,11 +38,7 @@ env:
     - CCACHE_COMPILERCHECK=none
     - CCACHE_NOCOMPRESS=1
     - CCACHE_MAXSIZE=512Mi
-    - >-
-      NPROC="`case ${TRAVIS_OS_NAME} in
-      osx)   sysctl -n hw.activecpu ;;
-      linux) nproc ;;
-      esac`"
+    - NPROC="`nproc`"
     # JOBS and SETARCH are overridden when necessary; see below.
     - JOBS=-j$((1+${NPROC}))
     - SETARCH=
@@ -62,27 +58,6 @@ env:
     script:
       - $SETARCH make -s test TESTOPTS="${TESTOPTS=$JOBS -q --tty=no}"
 
-  - &osx-clang
-    compiler: clang
-    before_install:
-      - gem uninstall ntlm-http plist -x --force
-      # `brew update` on Travis randomly hangs
-      - |-
-        timeout 300 brew update ||
-        timeout 300 brew update ||
-        timeout 300 brew update
-      # Travis homebrew addon is unstable for now. Use `before_install` instead.
-      # https://travis-ci.community/t/osx-homebrew-addons-module-not-as-reliable-as-claimed/4054
-      - |-
-        brew install \
-          ccache \
-          gdbm \
-          gmp \
-          libffi \
-          openssl@1.1 \
-          zlib
-      - /usr/local/opt/openssl@1.1/bin/openssl version
-
   - &gcc-8
     compiler: gcc-8
     # # Not using addon to control retries
@@ -369,16 +344,6 @@ env:
       apt:
         sources: []
 
-  - &x86_64-darwin18
-    name: x86_64-darwin18
-    osx_image: xcode11
-    os: osx
-    <<: *osx-clang
-    <<: *cron-only
-    env:
-      - CONFIG_FLAG=--with-opt-dir=/usr/local/opt/openssl@1.1:/usr/local/opt/zlib
-      - TEST_ALL_OPTS="--timeout-scale=2.0 --excludes=\$(TESTSDIR)/excludes/_travis/osx"
-
   - &dependency
     name: Check dependencies in makefiles
     os: linux
@@ -415,7 +380,6 @@ env:
 
 matrix:
   include:
-    # To reduce time for finishing all jobs, running the slowest osx build first.
     # Build every commit:
     - <<: *x86_64-linux
     - <<: *i686-linux
@@ -429,7 +393,6 @@ matrix:
     - <<: *MSAN
     - <<: *UBSAN
     # Cron only:
-    - <<: *x86_64-darwin18
     - <<: *jemalloc
     - <<: *VM_CHECK_MODE
     - <<: *SUPPORT_JOKE
@@ -459,9 +422,7 @@ before_script:
   - "> .rbconfig.time"
   - sed -f tool/prereq.status template/Makefile.in common.mk > Makefile
   - date; make touch-unicode-files
-  - |-
-    [ ${TRAVIS_OS_NAME} != osx ] ||
-    ls -ltT tool/generic_erb.rb template/unicode_norm_gen.tmpl enc/unicode/data/*/ucd/.unicode-tables.time
+  - ls -ltT tool/generic_erb.rb template/unicode_norm_gen.tmpl enc/unicode/data/*/ucd/.unicode-tables.time
   - date; make -s $JOBS $UPDATE_UNICODE up
   - date; make -s $JOBS srcs
   - rm -f config.status Makefile rbconfig.rb .rbconfig.time

test/excludes/_travis/osx/IMAPTest.rb

@@ -1,3 +0,0 @@
-# https://travis-ci.org/ruby/ruby/jobs/444232675
-# randomly raises: Errno::EPROTOTYPE "Protocol wrong type for socket"
-exclude(:test_imaps_post_connection_check, 'This test randomly fails with OpenSSL 1.1.1 on Travis osx build')

test/excludes/_travis/osx/TestGemRemoteFetcher.rb

@@ -1,4 +0,0 @@
-# https://travis-ci.org/ruby/ruby/jobs/444240249
-# raises: OpenSSL::SSL::SSLError "SSL_read: tlsv1 alert decrypt error"
-exclude(:test_do_not_allow_invalid_client_cert_auth_connection,
-        'This test is failing with OpenSSL 1.1.1 on Travis osx build')

test/excludes/_travis/osx/TestProcess.rb

@@ -1,7 +0,0 @@
-# These tests randomly hang forever. For testing other things properly, skipped for now.
-
-# https://travis-ci.org/ruby/ruby/jobs/566409880
-exclude(:test_execopts_redirect_open_fifo_interrupt_raise, 'This test randomly hangs on Travis osx')
-
-# https://travis-ci.org/ruby/ruby/jobs/567547060
-exclude(:test_execopts_redirect_open_fifo_interrupt_print, 'This test randomly hangs on Travis osx')

test/excludes/_travis/osx/TestWEBrickUtils.rb

@@ -1,3 +0,0 @@
-# https://travis-ci.org/ruby/ruby/jobs/454707326
-# maybe this test's timeout should be re-arranged for Travis osx
-exclude(:test_nested_timeout_outer, 'This test randomly fails on Travis osx')