internal/runtime/maps: remove type fields

internal/runtime/maps: remove type fields

Rather than storing the same type pointer in multiple places, just pass
it around.

For #54766.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest-swissmap
Change-Id: Ia6c74805c7a44125ae473177b317f16c6688e6de
Reviewed-on: https://go-review.googlesource.com/c/go/+/622377
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>