all: remove unnecessary return after skipping test

all: remove unnecessary return after skipping test

testing.Skip{,f} will exit the test via runtime.Goexit. Thus, the
successive return is never reached and can be removed.

Change-Id: I1e399f3d5db753ece1ffba648850427e1b4be300
Reviewed-on: https://go-review.googlesource.com/74990
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>