maps: replace slicesEqual with slices.Equal

maps: replace slicesEqual with slices.Equal

Change-Id: I15aeef9c6d3ddc80644a14d6266d21a8515ede4e
Reviewed-on: https://go-review.googlesource.com/c/go/+/483156
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>