modesetting: Remove obsolete, unused msPixmapPrivate declaration and macro

When the pixmapPrivateKeyRec was moved from a global to being embedded
inside the drmmode_rec these 2 where missed, clean them up.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

hw/xfree86/drivers/modesetting/drmmode_display.h

@@ -236,10 +236,6 @@ typedef struct _msPixmapPriv {
     Bool notify_on_damage; /* if sink has requested damage notification */
 } msPixmapPrivRec, *msPixmapPrivPtr;
 
-extern DevPrivateKeyRec msPixmapPrivateKeyRec;
-
-#define msPixmapPrivateKey (&msPixmapPrivateKeyRec)
-
 #define msGetPixmapPriv(drmmode, p) ((msPixmapPrivPtr)dixGetPrivateAddr(&(p)->devPrivates, &(drmmode)->pixmapPrivateKeyRec))
 
 Bool drmmode_is_format_supported(ScrnInfoPtr scrn, uint32_t format,