The Texinfo version of the tracing options documentation has now
been deleted, so we can remove the now-redundant comment at the top
of the rST version that was reminding us that the two should be
kept in sync.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200306171749.10756-8-peter.maydell@linaro.org