iio: max44000: remove unused variable max44000_alstim_shift

Variable max44000_alstim_shift is defined but is never used hence it is
redundant and can be removed. This variable has been like this since
the driver was added back in 2016.

Cleans up clang warning:
warning: 'max44000_alstim_shift' defined but not used [-Wunused-const-variable=]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>