[Ada] Remove xgnatugn in doc generation

[Arnaud Charlet <charlet at adacore dot com>]

[
Note: the previous related change was rejected due to a too large
patch file attached. The mail was:

This is another step in removing VMS code, and also a step in converting the
documentation to the rest/sphinx format.

The xgnatugn preprocessing step will also soon be removed.

2014-08-01  Ben Brosgol  <brosgol@adacore.com>

        * gnat_ugn.texi, projects.texi, xgnatugn.adb: Removed all VMS
        conditionalization from gnat_ugn.texi and projects.texi, and updated
        (considerably simplified) xgnatugn.adb, to be removed soon.
]

Following up from previous doc clean ups (see above), this change removes the
preprocessing step needed to generate gnat_ugn.info.

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

2014-08-01  Arnaud Charlet  <charlet@adacore.com>

	* ug_words, xgnatugn.adb, gcc-interface/Make-lang.in: Remove
	xgnatugn.adb and ug_words.

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