Purge upstream caches when deleting file assets.

Inform upstream caches when a file asset is deleted by calling
SquidUpdate::purge(). This helps prevent unintentional information leakage in
the event that the asset is being deleted for reasons of license or content.

Bug: 51064
Change-Id: Ibf57e8f32d7dd04b8508a69706be5a40b49e5abd