Compatibility shims for OC_Mount_Config

The following functions have been removed:
 - addMountPoint()
 - removeMountPoint()
 - movePersonalMountPoint()

registerBackend() has been rewritten as a shim around BackendService,
allowing legacy code to interact with the new API seamlessly

addMountPoint() was already disconnected from all production code, so
this commit completes the job and removes the function itself, along
with disconnecting and removing related functions. Unit tests have
likewise been removed.

getAbsoluteMountPoints(), getSystemMountPoints() and
getPersonalMountPoints() have been rewritten to use the StoragesServices