ada: Move formal hash tables from gnat repository to the SPARK library

ada: Move formal hash tables from gnat repository to the SPARK library

The formal containers have been part of the SPARK library for some
time now. However, some units used only by these containers are still
part of the gnat repository. Move them to the SPARK library.

gcc/ada/ChangeLog:

	* Makefile.rtl: Remove references to moved units.
	* libgnat/a-chtgfk.adb: Removed.
	* libgnat/a-chtgfk.ads: Removed.
	* libgnat/a-chtgfo.adb: Removed.
	* libgnat/a-chtgfo.ads: Removed.
	* libgnat/a-cohata.ads (Generic_Formal_Hash_Table_Types): Removed.