Partial automatic cleanup driven by Dominik Honnef's unused tool.

As _lookup now only has one caller, merge it into the caller and remove
the conditional create logic.

Change-Id: I2ea354d9d4b32a19905271eca74725231b6d8a93
Reviewed-on: https://go-review.googlesource.com/20589
Run-TryBot: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>