ramips: remove memory node for ZBT MT7621 devices

It's known that ZBT sells 256M variants of these routers. As a result,
our images won't be able to boot on these routers.
This commit removes memory node for them. With previously backported
memory detection patch, kernel is able to detect memory size itself.

Fixes: FS#3053
Signed-off-by: Chuanhong Guo <gch981213@gmail.com>

target/linux/ramips/dts/ZBT-WE3526.dts

@@ -9,11 +9,6 @@
 	compatible = "zbtlink,zbt-we3526", "mediatek,mt7621-soc";
 	model = "ZBT-WE3526";
 
-	memory@0 {
-		device_type = "memory";
-		reg = <0x0 0x1c000000>, <0x20000000 0x4000000>;
-	};
-
 	chosen {
 		bootargs = "console=ttyS0,115200";
 	};

target/linux/ramips/dts/ZBT-WG2626.dts

@@ -16,11 +16,6 @@
 		led-upgrade = &led_status;
 	};
 
-	memory@0 {
-		device_type = "memory";
-		reg = <0x0 0x1c000000>, <0x20000000 0x4000000>;
-	};
-
 	chosen {
 		bootargs = "console=ttyS0,115200";
 	};

target/linux/ramips/dts/ZBT-WG3526.dtsi

@@ -13,11 +13,6 @@
 		led-upgrade = &led_status;
 	};
 
-	memory@0 {
-		device_type = "memory";
-		reg = <0x0 0x1c000000>, <0x20000000 0x4000000>;
-	};
-
 	chosen {
 		bootargs = "console=ttyS0,115200";
 	};