nanobsd: remove sparc64 embedded example

Remove the qemu sparc64 example. It was only ever compile tested since
qemu had issues booting FreeBSD/sparc64. Also remove obsolete info about
armv5 configs removed long ago.

Sponsored by: Netflix

(cherry picked from commit 25a66f1fb177e0f3628f92f73b6ecf48a305d230)