iio: core: drop iio_get_time_res()

This function was introduced with the ability to pick a clock.
There are no upstream users so presumably it isn't as obviously useful
as it seemed at the time.  Hence drop it.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20220220163327.424696-1-jic23@kernel.org