ci: drop macOS from R-CMD-check (MOMF/rgl requires XQuartz)

Author: mhurtado13

.github/workflows/R-CMD-check.yaml

@@ -20,7 +20,6 @@ jobs:
       fail-fast: false
       matrix:
         config:
-          - {os: macos-latest,   r: 'release'}
           - {os: ubuntu-latest,  r: 'release'}
           - {os: ubuntu-latest,  r: 'devel', http-user-agent: 'release'}