[Ada] Remove doc for language version switches

[Pierre-Marie de Rodat]

Remove documentation for Ada language version switches, and note that
they are no longer needed.  These tools now silently ignore such
switches, and process the file correctly no matter what version of Ada
is used.

Tested on x86_64-pc-linux-gnu, committed on trunk

2019-08-12  Bob Duff  <duff@adacore.com>

gcc/ada/

	* doc/gnat_ugn/gnat_utility_programs.rst (gnatmetric, gnatpp,
	gnatstub): Remove documentation for Ada language version
	switches, and note that they are no longer needed.
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