rb532: drop target

rb532: drop target

This target is still on kernel 4.14, and recent attempts to move it to
kernel 5.4 have not led to success. The device tester reported that it
wouldn't boot with the following messages:

From sysupgrade:

  Press any key within 4 seconds to enter setup....
  loading kernel from nand... OK
  setting up elf image... OK
  jumping to kernel code

At this point the system hangs.

From CompactFlash:

  Press any key within 4 seconds to enter setup....
  Booting CF
  Loading kernel... done
  setting up elf image... kernel out of range kernel loading failed

The tester reported that the same was observed with current master
(kernel 4.14) as well. This looks like some kernel size restriction.

Since this target is quite old and only supports one device, and since
nobody else seemed interested in working on this for quite some time,
I decided to not put further work into analyzing the problem and drop
this together with the other 4.14-only targets.

Patchwork series:
https://patchwork.ozlabs.org/project/openwrt/list/?series=197066&state=*

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