go/internal/typeparams: melt it down

go/internal/typeparams: melt it down

This package is no longer needed now that typeparams
are unconditionally enabled.  Its declarations have been
moved into the go/{types,parser} packages.

Change-Id: Ife79a17eb9d29b076cabbf8a4b2ff2aea5edfc8f
Reviewed-on: https://go-review.googlesource.com/c/go/+/621640
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>