There is little obvious use case for a regualtor driver to know if it is
possible to vary voltages at all by itself.  If a consumer needs to
limit what voltages it tries to set based on the system configuration
then it will need to enumerate the possible voltages, and without that
even if it is possible to change voltages that doesn't mean that
constraints or other consumers will allow whatever change the driver is
trying to do at a given time.  It doesn't even indicate if _set_voltage()
calls will work as noop _set_voltage() calls return success.

There were no users of this API that weren't abusing it and now they're
all gone so remove the API.
Signed-off-by: Mark Brown <broonie@kernel.org>