gl-renderer: remove platform_attribs

No caller ever used anything but NULL here, so just use NULL to simplify code.

In fact, no EGL platform defined today even defines any platform attributes
except the X11 platform for choosing a non-default SCREEN.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

libweston/backend-x11/x11.c

@@ -1809,7 +1809,7 @@ init_gl_renderer(struct x11_backend *b)
 		return -1;
 
 	ret = gl_renderer->display_create(b->compositor, EGL_PLATFORM_X11_KHR,
-					  (void *) b->dpy, NULL,
+					  (void *) b->dpy,
 					  gl_renderer->opaque_attribs, NULL, 0);
 
 	return ret;