go/types, types2: use Checker.implements in operand.assignableTo

go/types, types2: use Checker.implements in operand.assignableTo

Now that we have the detailed error reporting in Checker.implements
we don't need it anymore in operand.assignableTo and can simply call
Checker.implements. This also more directly matches the spec.

For #50646.

Change-Id: Ic44ced999c75be6cc9edaab01177ee0495147ea1
Reviewed-on: https://go-review.googlesource.com/c/go/+/381435
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>