docs/conf.py: Disable unused _static directory

We don't yet have any custom static files, so disable this
config file setting to avoid a warning from sphinx about
not being able to find the directory.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190305172139.32662-5-peter.maydell@linaro.org
Message-id: 20190228145624.24885-5-peter.maydell@linaro.org