* ar71xx-generic: only create manifest alias for Rocket M5

This follow up the discussion done in #2070 by not  creating a symlink
for the Rocket M5. Images for the Rocket M2 can still be flashed on a
Rocket M5.

This change will prevent the Rocket M5 from appearing in Firmware
selectors. Existing devices will still receive updates, as the device
name is still referenced for the device name expected by the M5.

Closes #2070

* docs: remove Rocket M5 from supported devices