go/types: remove unused gcCompatibilityMode flag (cleanup)
This is a port of CL 339831 to go/types.
gcCompatibilityMode is unused, and x/tools/go/types no longer exists, so
delete it.
Change-Id: I886d8c24b7aa6511934ac78549f07a88a18e950b
Reviewed-on: https://go-review.googlesource.com/c/go/+/342429
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>