app/appengine, app/key: delete

The appengine command is no longer needed since
the coordinator is serving the build dashboard.

The key package is now unused.

For golang/go#34744.

Change-Id: I74218bac6b7457434975d13842e59e150fc078be
Reviewed-on: https://go-review.googlesource.com/c/build/+/336792
Trust: Dmitri Shuralyov <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>

Author: dmitshur