fmt: remove stopAtNewline unused parameter

fmt: remove stopAtNewline unused parameter

This parameter is always false. The last occurrence of s.skipSpace(true)
was removed in mid-2015.

While at it, merge skipSpace into SkipSpace, since the latter was just a
wrapper without the parameter.

Found with github.com/mvdan/unparam.

Change-Id: I884ea4036f41234a898d6aeee515211c49b0b435
Reviewed-on: https://go-review.googlesource.com/52890
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Avelino <t@avelino.xxx>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>