Remove win-R-3.6 due to xgboost

Author: topepo

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

@@ -23,11 +23,7 @@ jobs:
       matrix:
         config:
           - {os: macOS-latest,   r: 'release'}
-
           - {os: windows-latest, r: 'release'}
-          # Use 3.6 to trigger usage of RTools35
-          - {os: windows-latest, r: '3.6'}
-
           # Use older ubuntu to maximise backward compatibility
           - {os: ubuntu-18.04,   r: 'devel', http-user-agent: 'release'}
           - {os: ubuntu-18.04,   r: 'release'}