ath79-nand: drop broken manifest alias (#3214)

fix 5deb3aae

This upgrade path never worked due to increase of the kernel partition size.
They dropped this name from SUPPORTED_DEVICES upstream:
https://github.com/openwrt/openwrt/commit/0d28e5d6440d2a37841a207f943e6e5a23172883



(cherry picked from commit 10b6d8e1198a5576e6aa8ec694c81674b7afb3fc)

Co-authored-by: Felix Baumann <felix.bau@gmx.de>