[build, ada, doc] Remove --enable-threads=gnat support

[Rainer Orth]

Arnaud Charlet <charlet@adacore.com> writes:

> Certainly looks good to me, although I probably can't officially approve it.

I think you can since this only affects Ada.  I'll wait a day or two if
anyone objects, then install the patch.

Thanks.
	Rainer

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Rainer Orth, Center for Biotechnology, Bielefeld University