ath79-generic: remove Ubiquiti AirMax targets

Remove due to ticket freifunk-gluon#2939

These devices risk being bricked in a way that can not be recovered
without running full TFTP or serial recovery, as the flash ends up in a
read-only state with no way of upgrading to a fixed version.

Thus, remove the support for the time being. Don't just flag them as
BROKEN, as this would deliberately brick these boards when installating
such an image.

Signed-off-by: David Bauer <mail@david-bauer.net>

docs/user/supported_devices.rst

@@ -128,10 +128,6 @@ ath79-generic
 
 * Ubiquiti
 
-  - NanoBeam 5AC 19 (XC)
-  - NanoBeam M5 (XW)
-  - NanoStation Loco M2/M5 (XW)
-  - NanoStation M2/M5 (XW)
   - UniFi AC Lite
   - UniFi AC LR
   - UniFi AC Mesh