[dev.typeparams] cmd/compile/internal/types2: remove code for implicit type arguments

The design draft doesn't support this anymore.

Also: Fixed a potential bug in the receiver unpack code
      (found by rfindley@).

Change-Id: Ic52eedc686adcb4d5a98884ad0134679c3685c13
Reviewed-on: https://go-review.googlesource.com/c/go/+/278853


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>