iio:common: removed unused functions outside st_sensors library

This patch remove st_sensors_get_sampling_frequency_avl and
st_sensors_get_scale_avl functions used only in
st_sensors_sysfs_sampling_frequency_avail and st_sensors_sysfs_scale_avail
sysfs functions.

Signed-off-by: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>