treewide: drop DEVICE_TYPE when used as device variable

DEVICE_TYPE is a target/subtarget variable, and it does not have
any effect when set in a device definition. It can only be set
in a target's or subtarget's Makefile.

Consequently, having it set anyway is misleading, so this drops
all cases.

This effectively reverts the following commits:
7a1497fd60 ("apm821xx: MBL: set DEVICE_TYPE to NAS")
5b4765c93a ("gemini: Classify Raidsonic NAS IB-4220-B as a NAS")
cdc6de460b ("gemini: D-Link DNS-313 is a NAS")

For the following commit, the variable was set when adding device
support:
27b2f0fc0f ("kirkwood: add support for Iomega Storcenter ix2-200")

Cc: Christian Lamparter <chunkeey@gmail.com>
Cc: Sungbo Eo <mans0n@gorani.run>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

target/linux/gemini/image/Makefile

@@ -171,7 +171,6 @@ define Device/dlink_dns-313
 	DEVICE_VENDOR := D-Link
 	DEVICE_MODEL := DNS-313 1-Bay Network Storage Enclosure
 	DEVICE_DTS := gemini-dlink-dns-313
-	DEVICE_TYPE := nas
 	DEVICE_PACKAGES := $(GEMINI_NAS_PACKAGES)
 	BLOCKSIZE := 1k
 	FILESYSTEMS := ext4
@@ -204,7 +203,6 @@ define Device/raidsonic_ib-4220-b
 	DEVICE_VENDOR := Raidsonic
 	DEVICE_MODEL := NAS IB-4220-B
 	DEVICE_DTS := gemini-nas4220b
-	DEVICE_TYPE := nas
 endef
 TARGET_DEVICES += raidsonic_ib-4220-b