staging: iio: light: use devm_iio_device_register instead iio_device_register

Replace iio_device_register with resource managed devm_iio_device_register in order
to ease the error path. Also delete the remove function since there is no need after
this change.

Signed-off-by: Ioana Ciornei <ciorneiioana@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>