* Remove some redundant returns
* Replace HasPrefix with TrimPrefix
* Remove some obviously dead code

Passes toolstash -cmp on std cmd.

Change-Id: Ifb0d70a45cbb8a8553758a8c4878598b7fe932bc
Reviewed-on: https://go-review.googlesource.com/105017
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>