ramips-mt7620: remove factory images for Archer C20 v1

Upstream removed the factory images for the Archer C20 v1 as they
potentially brick the device. Remove them from Gluon to avoid build
failures on the next OpenWrt 19.07 bump.

Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit 7d100bc4)