power: bq27xxx_battery: Remove unneeded dependency in Kconfig

I2C is only required when using the config item BATTERY_BQ27XXX_I2C which
already depends on the I2C subsystem, remove the unneeded dependency from
BATTERY_BQ27XXX.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Acked-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>