Bug 1679923 - Upgrade rustc to 1.49 beta 2 for arm64 mac builds. r=firefox-build-system-reviewers,dmajor

At the same time, remove the macosx64-rust-beta toolchain, unused since
bug 1678154.

Differential Revision: https://phabricator.services.mozilla.com/D98277

taskcluster/ci/toolchain/rust.yml

@@ -124,7 +124,7 @@ linux64-rust-macos-beta:
         symbol: TL(rust-macos-beta)
     run:
         arguments: [
-            '--channel', 'beta-2020-11-18',
+            '--channel', 'beta-2020-11-26',
             '--host', 'x86_64-unknown-linux-gnu',
             '--target', 'x86_64-unknown-linux-gnu',
             '--target', 'x86_64-apple-darwin',
@@ -190,17 +190,6 @@ macosx64-rust-1.47:
         ]
         toolchain-alias: macosx64-rust
 
-macosx64-rust-beta:
-    treeherder:
-        symbol: TM(rust-beta)
-    run:
-        arguments: [
-            '--channel', 'beta-2020-11-18',
-            '--host', 'x86_64-apple-darwin',
-            '--target', 'x86_64-apple-darwin',
-            '--target', 'aarch64-apple-darwin',
-        ]
-
 mingw32-rust-1.47:
     treeherder:
         symbol: TMW(rust)