GHA: no need to check on R 3.6

In fact, we cannot run the check.

Author: gaborcsardi

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

@@ -27,8 +27,6 @@ jobs:
           - {os: windows-latest, r: 'devel'}
           - {os: windows-latest, r: 'next'}
           - {os: windows-latest, r: 'release'}
-          # Use 3.6 to trigger usage of RTools35
-          - {os: windows-latest, r: '3.6'}
           # use 4.1 to check with rtools40's older compiler
           - {os: windows-latest, r: '4.1'}