docs: Stop building qemu-doc

Stop building the old texinfo qemu-doc; all its contents are
now available in the Sphinx-generated manuals and manpages.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20200228153619.9906-32-peter.maydell@linaro.org