ramips: remove duplicate DEVICE_PACKAGES for TP-Link Archer C20i

DEVICE_PACKAGES is specified twice for the same device. Remove the
first (=older) assignment.

Fixes: 40692f0f ("ramips: mt7620: select only the matching mt76 driver")

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>