backend/headless: stop picking a DRM FD

Sometimes the headless backend is used standalone with the Pixman
renderer, sometimes it's used together with another backend which
has already picked a DRM FD. In both of these cases it doesn't make
sense to pick a DRM FD.

Broadly speaking the headless backend doesn't really care which DRM
device is used for the buffers it receives. So it doesn't really
make sense to tie it to a particular DRM device.

Let the backend users (e.g. wlr_renderer_autocreate) open an arbitrary
DRM FD as needed instead.

backend/headless/output.c

@@ -2,7 +2,6 @@
 #include <stdlib.h>
 #include <stdio.h>
 #include <wlr/interfaces/wlr_output.h>
-#include <wlr/render/wlr_renderer.h>
 #include <wlr/util/log.h>
 #include "backend/headless.h"
 #include "util/signal.h"