power_supply: max8998: Use devm_power_supply_register

Use managed resource function devm_power_supply_register instead
of power_supply_register to simplify the error path by allowing
unregister to happen automatically on error. To be compatible with
the change, replace various gotos by direct returns and remove
unneeded label err.

Also, remove max8998_battery_remove as it is now redundant.

Signed-off-by: Vaishali Thakkar <vthakkar1994@gmail.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>