iio: imu: st_lsm6dsx: make IIO_CHAN_INFO_SCALE shared by type

in_accel_x_scale, in_accel_y_scale and in_accel_z_scale are always
the same. The scale is still defined to be in "info_mask_separate".

Userspace (iio-sensor-proxy and others) is not used to that and only
looks for "in_accel_scale" for the scaling factor to apply.

Change IIO_CHAN_INFO_SCALE from being separate in all channel to be
shared by type.

This removes in_accel_x_scale, in_accel_y_scale and in_accel_z_scale and
makes available in_accel_scale.

Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
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