Revert "[gh-actions] Update macos.yml, add brew update step"

This reverts commit 0f33a896de.

Reason is that using newest homebrew seems to make the workflow very unstable.

.github/workflows/macos.yml

@@ -14,9 +14,6 @@ jobs:
     # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
     - uses: actions/checkout@v2
 
-    - name: Update brew
-      run: brew update
-
     - name: Install build tools
       run: brew install automake autoconf libtool pkg-config