regulator: Remove some unused variables from wm831x DCDCs

These became unused with the IRQ removal patch, I'm fairly sure that a
patch was sent earlier by someone else but it doesn't seem to have been
applied and I don't have a copy sitting around any more.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>