cns3xxx: drop target

cns3xxx: drop target

This target has not been updated to 5.4 yet, and the only person
trying it (Koen) decided to retreat based on the following reasons:

- The target is not DT-aware at all

- The huge amount of effort required

- The SoC itself reached EoL at Cavium for some time now

- Upstream removed some important parts as it's also slowly getting EoL
  over there

- The commercial product that used this will fade out shortly

- The amount of download for this binary suggest that the target is not
  that popular

Since nobody has picked up the work since then, and this is the last
remaining 4.19-only target, finally drop it now.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>