ramips: drop useless label-mac-device for RouterBOARD 750Gr3

With the new driver, MAC addresses are not set up in DTS anymore,
and therefore label-mac-device will be useless there.

Setup is done properly in 02_network, so this just removes the
obsolete alias.

Fixes: 5e50515fa6 ("ramips/mt7621: mikrotik: don't use
mtd-mac-address in DTS")

Suggested-by: John Thomson <git@johnthomson.fastmail.com.au>
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

target/linux/ramips/dts/mt7621_mikrotik_routerboard-750gr3.dts

@@ -15,7 +15,6 @@
 		led-failsafe = &led_usr;
 		led-running = &led_usr;
 		led-upgrade = &led_usr;
-		label-mac-device = &gmac0;
 	};
 
 	chosen {