[Ada] Remove obsolete uses of Unchecked_Deallocation from Ada 83

[Ada] Remove obsolete uses of Unchecked_Deallocation from Ada 83

In Ada 83 the Unchecked_Deallocation was a top-level unit; since Ada 95
it is an obsolete renaming of Ada.Unchecked_Deallocation. GNAT doesn't
yet warn about uses of these obsolete renamings, but it still seems
better to avoid them.

Cleanup before adding a new instance of Unchecked_Deallocation.
Offending occurrences found with grep.

gcc/ada/

	* butil.adb, sem.adb, sinput.adb, types.ads, xref_lib.adb:
	Replace uses of Unchecked_Deallocation with
	Ada.Unchecked_Deallocation.
	* doc/gnat_ugn/gnat_and_program_execution.rst: Likewise for the
	documentation; fix casing of GNAT.IO.
	* gnat_ugn.texi: Regenerate.