power: pm8921-charger: remove enabling of input regulation

There is no need to enable input voltage regulation every time usb
is inserted or removed. The input voltage regulation is never disabled.

Remove the enable voltage regulation function and the calls to it.

Change-Id: I9c87282ba5819a76a6f6697a5e9de6c92d74a665
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>