remove osx and linux actions (#10209)

.github/workflows/main.yml

@@ -17,17 +17,11 @@ jobs:
       matrix:
         # Useful info: https://help.github.com/en/articles/workflow-syntax-for-github-actions
         include:
-          - name: ubuntu
-            os: ubuntu-18.04
-            python-version: 3.6
           - name: windows-msvc
             os: windows-2019
             python-version: 3.8
             # Can be 'msvc' or 'clang-cl'
             config: msvc
-          - name: macos
-            os: macos-10.15
-            python-version: 3.6
     env:
       BAZEL_CONFIG: ${{ matrix.config }}
       PYTHON: ${{ matrix.python-version }}