iio: adc: pac1934: remove unused struct 'samp_rate_mapping'

'samp_rate_mapping' has been unused since the original
commit 0fb528c8 ("iio: adc: adding support for PAC193x").

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Link: https://lore.kernel.org/r/20240522230457.478156-3-linux@treblig.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>