internal/runtime/maps: don't hash twice when deleting

internal/runtime/maps: don't hash twice when deleting

                     │  baseline   │             experiment              │
                     │   sec/op    │   sec/op     vs base                │
MapDeleteLargeKey-24   312.0n ± 6%   162.3n ± 5%  -47.97% (p=0.000 n=10)

Change-Id: I31f1f8e3c344cf8abf2e9eb4b51b78fcd67b93c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/625906
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>