Motor tries to write to read-only PVs

[dperl-dls]

Mea culpa, I misunderstood what _backend.put did, and so Motor.stop() is broken, as it tries to write to the RBV PV.

Removing the line that does this won't break anything which currently exists, but we should have a new issue for the interaction of stop() with any callbacks or observe_values on motor moves which don't have explicit mechanisms for escaping as the default set() status does.