Enhancement: Clear removed snapshots, index and pack files from the local cache

Restic did not clear removed snapshots from the cache after the `forget` 
operation; only indexes and pack files were removed automatically.  
Restic now automatically clears removed indexes, packs and snapshots from the
local cache.

https://github.com/restic/restic/issues/4934
https://github.com/restic/restic/pull/4981