power: supply: ab8500: remove unused static local variable

cpp_check reports
[drivers/power/supply/ab8500_chargalg.c:493]: (style) Variable 'ab8500_chargalg_ex_ac_enable_toggle' is assigned a value that is never used.

From inspection, this variable is never used. So remove it.

Fixes: 6c50a08d ("power: supply: ab8500: Drop external charger leftovers")
Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Chen Lifu <chenlifu@huawei.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>