Remove the deprecated gc package

The `gc` package is now deprecated (see its Hackage page). As
co-maintainer, I'd like to remove it from Stackage.

build-constraints.yaml

@@ -809,7 +809,6 @@ packages:
         - fixed
         - folds
         - free
-        - gc
         - gl
         - graphs
         - half
@@ -6446,7 +6445,6 @@ skipped-tests:
     - dotenv
     - enum-subset-generate
     - folds
-    - gc
     - generic-lens
     - geojson
     - github-types