regulator: fixed: remove QTI_FIXED_REGULATOR definition

Now that all fixed regulator devices which utilize proxy-consumer
properties have switched to be qti-fixed-regulator devices, the
definition of QTI_FIXED_REGULATOR can be removed from fixed.c.

Change-Id: I5847ead86723b63771d69f4076eafd26139baa80
Signed-off-by: David Collins <collinsd@codeaurora.org>