[dev.typeparams] go/types: remove unused *Checker arguments (cleanup)

[dev.typeparams] go/types: remove unused *Checker arguments (cleanup)

This is a straightforward port of CL 331512 to go/types. API usage in
methodset.go was also updated.

Change-Id: I6701265c9d2ae40eb9aa0ea5f00c98ce3516edab
Reviewed-on: https://go-review.googlesource.com/c/go/+/335009
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>