ramips: fix Archer C20 sysupgrade

The sysupgrade image failed the check due to the wrong string in the
supported devices. This patch provides the correct name by dropping the
SUPPORTED_DEVICES to use the default generated name.

Signed-off-by: Steffen Förster <steffen@chemnitz.freifunk.net>
[drop the SUPPORTED_DEVICES, the old name was never used in a release]
Signed-off-by: Mathias Kresin <dev@kresin.me>

target/linux/ramips/image/mt7620.mk

@@ -516,7 +516,6 @@ TARGET_DEVICES += tplink_c2-v1
 define Device/tplink_c20-v1
   $(Device/Archer)
   DTS := ArcherC20v1
-  SUPPORTED_DEVICES := c20v1
   TPLINK_FLASHLAYOUT := 8Mmtk
   TPLINK_HWID := 0xc2000001
   TPLINK_HWREV := 0x44