arch: remove unicore32 port

The unicore32 port do not seem maintained for a long time now, there is no
upstream toolchain that can create unicore32 binaries and all the links to
prebuilt toolchains for unicore32 are dead. Even compilers that were
available are not supported by the kernel anymore.

Guenter Roeck says:

  I have stopped building unicore32 images since v4.19 since there is no
  available compiler that is still supported by the kernel. I am surprised
  that support for it has not been removed from the kernel.

Remove unicore32 port.

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Guenter Roeck <linux@roeck-us.net>