go/types, types2: remove setDefType and most def plumbing

go/types, types2: remove setDefType and most def plumbing

CL 722161 replaced the setDefType mechanism with boundaries on composite
literals, removing the need to pass the def argument in all but 1 case.

The exception is interface types, which use def to populate the receiver
type for better error messages.

Change-Id: Ic78c91238588015153f0d22790be5872a01c5f63
Reviewed-on: https://go-review.googlesource.com/c/go/+/723920
Auto-Submit: Mark Freeman <markfreeman@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>