cmd/compile/internal/types2: remove targs from substMap

Now that we always capture targs when constructing an instance, we no
longer need to pass them via the substMap. This simplifies the code and
resolves a TODO.

Change-Id: I592dccaeb89c7cc31ac037d919137bb762820365
Reviewed-on: https://go-review.googlesource.com/c/go/+/341859


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>