leds: leds-cobalt-qube: Use devm_led_classdev_register

Use resource-managed function devm_led_classdev_register instead
of led_classdev_register to make the error-path simpler.

To be compatible with the change, goto is replaced with direct
return, unneeded label err_null is dropped and unnecessary variable
retval is removed. Also, remove redundant cobalt_qube_led_remove.

Signed-off-by: Vaishali Thakkar <vthakkar1994@gmail.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>