power: supply: charger-manager: Remove cm_notify_event function

cm_notify_event() was introduced to get an event associated with the
battery status externally (ie in board files), but no one ever used it.
Moreover it makes charger manager driver more complicated. Drop the
function and all data related to it to simplify the driver.

Signed-off-by: Jonghwa Lee <jonghwa3.lee@samsung.com>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Jonathan Bakker <xc-racer2@live.ca>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>