[dev.typeparams] cmd/compile/internal/types2: remove unused gcCompatibilityMode flag (cleanup)

This flag is not needed by types2 (and possibly can also be removed
from go/types). Removed some unnecessary comments along the way.

Updates #46174.

Change-Id: I1a7a99f724205a084d1c9850bce6f6f5d33f83ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/339831


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>