go/parser: use (*Scanner).End instead of scannerhooks

go/parser: use (*Scanner).End instead of scannerhooks

Also drop the scannerhooks code, as it is now unused.

Updates #74958
Updates #76031

Change-Id: I5a0cb8a7fe954e40ce40b8406aed0d3f6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/738701
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>