This legacy option provided the ability to fail on doc warnings.
However, this functionality is broken in recent releases and now exists
in Sphinx itself (since 1.5.0). Rather that fixing it and causing a
whole load of doc build errors introduced in the time since this option
was broken, remove it, preferring the new Sphinx option instead.
This allows us to remove a lot of test code which is essentially testing
Sphinx functionality only now, based on the assumption that Sphinx do
adequate testing themselves.
Change-Id: Ia4b6adefcd437cb1ceb4558b004c17359df2486d
72e8e42a34