app/appengine: remove ancient performance/benchmarking code

This was the performance/benchmark code from ~three generations ago.

It's unused and unmaintained. It broke when we moved from mercurial to
git, IIRC.

I'm attempting to modernize this code (for golang/go#34744) but it'd
be easier if there's less code to deal with.

Updates golang/go#34744

Change-Id: Ib4999830b05df9ffad9b46964022325404350b47
Reviewed-on: https://go-review.googlesource.com/c/build/+/208319
Reviewed-by: Bryan C. Mills <[email protected]>
Reviewed-by: Andrew Bonventre <[email protected]>

Author: bradfitz

app/appengine/app.yaml

@@ -5,7 +5,7 @@ handlers:
   - url: /static
     static_dir: app/appengine/static
     secure: always
-  - url: /(init|buildtest|key|perflearn|_ah/queue/go/delay)
+  - url: /(init|buildtest|key|_ah/queue/go/delay)
     script: auto
     login: admin
     secure: always