ath79: rb912: remove unused property

ar934x does not define property 'rgmii-enabled' in the parsing code
remove it

Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>

target/linux/ath79/dts/ar9342_mikrotik_routerboard-912uag-2hpnd.dts

@@ -198,7 +198,6 @@
 	gmac-config {
 		device = <&gmac>;
 		rgmii-gmac0 = <1>;
-		rgmii-enabled = <1>;
 		rxd-delay = <1>;
 		txd-delay = <1>;
 	};