internal/runtime: remove math.Mul64

internal/runtime: remove math.Mul64

internal/runtime/math.Mul64 is a copy of math/bits.Mul64 and redundant.

Change-Id: I4dd2ab531a32da97839c6b45cf90df6430811967
GitHub-Last-Rev: 1a73e16049ee346ccfa8f052856e49e10e202d70
GitHub-Pull-Request: golang/go#77187
Reviewed-on: https://go-review.googlesource.com/c/go/+/736500
Auto-Submit: Keith Randall <khr@google.com>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>