power: pm8921-bms: remove reading rbatt from bms

Since battery resistance is found to be changing w.r.t. state of charge,
temperature and time since a load change happened, it is best if rbatt
is estimated using learnt tables.

Remove all the code that attempts to read battery resistance from BMS.

Change-Id: Id8d84d610df0bcca79d7e42ffae572c0898d63c6
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>