Simplify map key literals in "gofmt -s"

Fixes #16461.

Change-Id: Ia61739b34a30ac27f6696f94a98809109a8a7b61
Reviewed-on: https://go-review.googlesource.com/25530
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>