BlobCache: remove the mutex locking

This change removes the mutex from the BlobCache class.  The caller must
be responsible for thread synchronization in order to properly implement
the Flattenable interface, which is coming soon.  Otherwise would be the
potential for the cache contents to change between the call to the
getFlattenedSize and flatten methods.  Because the caller must do this
synchronization anyway there's no reason to also some synchronization
inside BlobCache.

Change-Id: Ie1f4f6f82b78744f46a41ce863cac0cad276a20e