app/appengine: move some code, remove some variables
app/appengine: move some code, remove some variables

From comment by Alex on at
https://go-review.googlesource.com/c/build/+/210997/5/app/appengine/ui.go#69

Change-Id: I5aa3ee0ee0d0c2f434c8a78ae5127ebb089a60ec
Reviewed-on: https://go-review.googlesource.com/c/build/+/211078
Reviewed-by: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
Run-TryBot: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/build,golang/build,golang/build,golang/build