cmake: remove deprecation_warnings.cmake

With support for deprecating Kconfig symbols within the Kconfig tree
itself, there is no reason to have a secondary location to maintain
for information that is based on a Kconfig setting in first place.

Better to directly do `select DEPRECATED` on the symbol itself, than to
maintain a secondary list with messages in CMake.

Hence removing this feature in CMake.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>