regulator: qpnp-labibb: remove unused global functions

There are some global functions in LABIBB regulator driver which
are not used by display panel drivers, or any other clients.
Remove the definition and declaration of these functions.

Change-Id: Ifa077fc024c675345c9694a39714d9e68f64ddf9
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>