Makefile: Remove code to smooth transition to config.status

When commit bdf523e6 made configure generate config.status, it
added a fallback to Makefile to smooth the transition, with a TODO
"code can be removed after QEMU 1.7."  It's been more than five years.
Remove it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190528082308.22032-2-armbru@redhat.com>