power: supply: sbs-battery: Don't ignore the first external power change

A mechanism to ignore the first external power change notification was
put in place years ago to ignore the power_supply_register notification.

However, this doesn't apply to the current situation anymore, as the
first notification is always the result of a legitimate power change.

This removes this deprecated mechanism, which puts back the driver's
state machine to a sane state (an ignored first notification previously
caused a charging/discharging status inversion).

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>