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>

CMakeLists.txt

@@ -1852,8 +1852,6 @@ if(CONFIG_SOC_DEPRECATED_RELEASE)
   )
 endif()
 
-include(modules/deprecation_warnings.cmake)
-
 # In CMake projects, 'CMAKE_BUILD_TYPE' usually determines the
 # optimization flag, but in Zephyr it is determined through
 # Kconfig. Here we give a warning when there is a mismatch between the