[rubygems/rubygems] Remove very old compatibility code

https://github.com/rubygems/rubygems/commit/6e4bef758b

lib/rubygems/test_case.rb

@@ -280,9 +280,6 @@ class Gem::TestCase < Minitest::Test
 
   include Gem::DefaultUserInteraction
 
-  undef_method :default_test if instance_methods.include? 'default_test' or
-                                instance_methods.include? :default_test
-
   ##
   # #setup prepares a sandboxed location to install gems.  All installs are
   # directed to a temporary directory.  All install plugins are removed.