Remove Windows CI and add macOS X11 (XQuartz) install step

Co-authored-by: asgr <5617132+asgr@users.noreply.github.com>

Author: Copilot

.github/workflows/main.yml

@@ -21,7 +21,6 @@ jobs:
       matrix:
         config:
           - {os: macOS-latest,    r: 'release'}
-          - {os: windows-latest,  r: 'release'}
           - {os: ubuntu-latest,   r: 'devel', http-user-agent: 'release'}
           - {os: ubuntu-latest,   r: 'release'}
           - {os: ubuntu-latest,   r: 'oldrel-1'}
@@ -33,6 +32,12 @@ jobs:
     steps:
       - uses: actions/checkout@v2
 
+      - name: Install X11 on macOS
+        if: runner.os == 'macOS'
+        run: |
+          brew update
+          brew install --cask xquartz
+
       - uses: r-lib/actions/setup-pandoc@v2
 
       - uses: r-lib/actions/setup-r@v2