Remove the macos-10.15 build, which is the same as macos-latest.

Author: sunfishcode

.github/workflows/main.yml

@@ -130,7 +130,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        build: [stable, windows, macos, beta, ubuntu-16.04, ubuntu-20.04, macos-10.15, windows-2016]
+        build: [stable, windows, macos, beta, ubuntu-16.04, ubuntu-20.04, windows-2016]
         include:
           - build: stable
             os: ubuntu-latest
@@ -150,9 +150,6 @@ jobs:
           - build: ubuntu-20.04
             os: ubuntu-20.04
             rust: stable
-          - build: macos-10.15
-            os: macos-10.15
-            rust: stable
           - build: windows-2016
             os: windows-2016
             rust: stable