Delete App's buildpack cache when the app is deleted

Currently the buildpack cache associated with an app is not removed when
an app is deleted. This ends up filling up space on the blob store. For
deployments using local disk for the store this can lead to running out
of disk space.

This change adds methods to StagingsController and AppManager to allow
deleting the buildpack cache associated with an App, and calls them when
the app is deleted.