iio: imu: inv_mpu6050: use devm_* at init and delete remove

Use devm_* for iio_triggered_buffer_setup, iio_device_register,
iio_trigger_register. Delete unneeded inv_mpu6050_remove_trigger,
inv_mpu_core_remove, and inv_mpu_remove for spi driver.

Signed-off-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>