go/types: remove work-around for issue #26124

This work-around is not needed anymore now that method
signatures are type-checked separately from their receiver
base types: no artificial cycles are introduced anymore
and so there is no need to artificially cut them.

Updates #26124.

Change-Id: I9d50171f12dd8977116a5d3f63ac39a06b1cd492
Reviewed-on: https://go-review.googlesource.com/c/139899


Reviewed-by: Alan Donovan <adonovan@google.com>