Fixes gcc '-Wunused-but-set-variable' warning:

drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_buffer.c: In function 'st_lsm6dsx_read_tagged_fifo':
drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_buffer.c:520:41: warning:
 variable 'gyro_sensor' set but not used [-Wunused-but-set-variable]

drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_buffer.c:520:28: warning:
 variable 'acc_sensor' set but not used [-Wunused-but-set-variable]

They are not used since commit 14c7c6e1 ("iio: imu: st_lsm6dsx: add
st_lsm6dsx_push_tagged_data routine")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>