ada: Print gnat diagnostics with the gnat command

ada: Print gnat diagnostics with the gnat command

Remove the support for printing the diagnostic report
in the compiler when -gnatd_E is used. Instead print it
when the "gnat --diagnostics" command is used.

gcc/ada/ChangeLog:

	* debug.adb: remove use case for -gnatd_E
	* errout.adb (Output_Messages): remove printing of
	the diagnostic repository when -gnatd_E is used.
	* gnatcmd.adb: Add support for printing the diagnostic repository.