power: supply: ab8500: Remove unused variable

Kernel test robot reported below warning ->
drivers/power/supply/ab8500_chargalg.c:790:13: warning:
variable 'delta_i_ua' set but not used [-Wunused-but-set-variable]

Remove unused variable delta_i_ua.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Souptick Joarder (HPE) <jrdr.linux@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>