treewide: drop DEVICE_TYPE when used as device variable

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:
7a1497fd601d ("apm821xx: MBL: set DEVICE_TYPE to NAS")
5b4765c93a1b ("gemini: Classify Raidsonic NAS IB-4220-B as a NAS")
cdc6de460bb4 ("gemini: D-Link DNS-313 is a NAS")

For the following commit, the variable was set when adding device
support:
27b2f0fc0fc5 ("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>