internal/runtime/maps: avoid passing unused key return

internal/runtime/maps: avoid passing unused key return

For #54766.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest-swissmap
Change-Id: Idee1e021e3cef8f0c031e8f06efbcf6e88918d8a
Reviewed-on: https://go-review.googlesource.com/c/go/+/622376
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>