Nuke polkitbackend library, localauthority backend and extension system

Nuke polkitbackend library, localauthority backend and extension system

Any backend can now be implemented in JavaScript (if so desired) so we
don't need any of this any more.

Note that the libpolkitbackend library was never declared stable (the
preprocessor symbol POLKIT_BACKEND_I_KNOW_API_IS_SUBJECT_TO_CHANGE had
to be defined) so removing it is not an API/ABI break.

Signed-off-by: David Zeuthen <davidz@redhat.com>