errors: don't display "warnings treated as errors"

This line doesn't add any meaningful information anymore, the appended
[-Werror=CATEGORY] is enough.  It is actually more insightful, as it
allows to distinguish warnings treated as errors from those that
aren't.  This line is also removed by gcc 4.8.

* src/complain.c (set_warnings_issued): The only action left was
checking if the error bit corresponding to the warning issued was set,
and that function was only called once. Therefore, remove it, and do
its job directly in the caller...
(complains): here.
* src/complains.h: Adjust.
* tests/input.at: Adjust.
* NEWS: Document this change.

Signed-off-by: Akim Demaille <akim@lrde.epita.fr>