ramips: remove factory image for TP-Link Archer C20 v1

ramips: remove factory image for TP-Link Archer C20 v1

Similarly to the Archer C2 v1, the Archer C20 v1 will brick when one
tries to flash an OpenWrt factory image through the TP-Link web UI.
The wiki page contains an explicit warning about this [1].

Disable the factory image altogether since it serves no purpose.

[1] https://openwrt.org/toh/tp-link/tp-link_archer_c20_v1#installation

Signed-off-by: Stijn Segers <foss@volatilesystems.org>
(backported from commit 0265cba40ad4f2b8ff4473ada123c35b53ffd97a)