power: pm8921-bms: remove vsense average irq

The vsense average irq is only meant for hardware debugging and
verification purposes. There is no need for the driver to request
that irq.

Remove requesting and handling the vsense average irq.

Change-Id: I8ec5a68cad573218a0b0bd0073277b76978417ff
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>