staging:iio: Remove periodic RTC trigger driver

With the recently introduced hrtimer based trigger we have a fully
functional replacement for the RTC timer trigger that is more flexible and
has a better interface to instantiate and manage the trigger instances. The
RTC trigger timer could only be instantiated using platform devices which
makes it unsuitable for modern devicetree based platforms, while the
hrtimer trigger has a configfs based interface that allows creating and
deletion of triggers at runtime.

In addition since a few years the periodic RTC timer is internally always
emulated using a hrtimer using the hrtimer interface directly will yield
the same timing precision. So using the RTC timer won't have any advantages
on this front either.

There is also no evidence that the periodic RTC trigger is currently
actually being used on any system. So considering all this remove the
driver. Also remove the related item from the TODO list.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>