staging: comedi: Remove unused variable ‘min_full_scale’ and function 'get_min_full_scales'

The variable ‘min_full_scale’ and function 'get_min_full_scales' are
not used, So delete them.

Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Bixuan Cui <cuibixuan@huawei.com>
Link: https://lore.kernel.org/r/20210514085214.53941-1-cuibixuan@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>