go/types: remove the concept of finals

Checker.finals and the corresponding atEnd were added in CL 191418 as a
mechanism to postpone interface type comparison until after all
interfaces were complete. In the intervening CL 195837 we've adopted a
convention of ensuring that interfaces are complete before comparing
them. Since then we've also added the additional case of expansion for
lazily resolving syntax.

Checker.later defers resolution of types until points in the checking
pass where all reachable types can be fully type checked, so the concept
of finals should no longer be necessary.

Change-Id: I58818c1a6b605dccc9b0ecb3a1f6859c138175d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/299590


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>