services/internal/pathperms: delete unused PathStore locks
services/internal/pathperms: delete unused PathStore locks

Fix a TODO in the pathperms package: currently the pathperms library
never deletes mutexes controlling access to permission lists even if
the corresponding path is no longer in use. This CL changes the
pathperms locking to remove unnecessary mutexes.

Change-Id: I2608b8fcabf9dac1a4e84b044224a4661c3c9a48

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref