power: pm8921-charger: remove unused charging enable/disable api

This api could interfere with the internal working of the charger.
The end of charge detection is broken in hardware, the software workaround
relies on the enable/disable of the charging function. We don't want
this to be changed via some external api.

CRs-Fixed: 417243
Change-Id: Idc4ceb7973d1a424834f8c2a4bc5916ef7b1e961
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>