[dev.typeparams] cmd/compile/internal/types2: implement delete(m, k) where m...

[dev.typeparams] cmd/compile/internal/types2: implement delete(m, k) where m is of type parameter type

Change-Id: Iaf33c15128af911b6101df9885cb8b5a8495b942
Reviewed-on: https://go-review.googlesource.com/c/go/+/333729


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>