ci: drop BC

Author: Bogdanp

.github/workflows/ci.yml

@@ -6,10 +6,7 @@ jobs:
     strategy:
       matrix:
         racket-version: ['8.6', '8.7', 'current']
-        racket-variant: ['BC', 'CS']
-        exclude:
-          - racket-version: current
-            racket-variant: BC
+        racket-variant: ['CS']
     name: Build & Test on Racket ${{ matrix.racket-version }} (${{ matrix.racket-variant }})
     steps:
       - uses: actions/checkout@master