Remove picobsd build scripts and the remaining tinyware

Postings were sent to -arch@ on 2019/09/13 and 2019/10/01, proposing and
confirming a removal of these scripts on 2019/10/31, due to significant work
needed to bring this into the modern world and nobody having done this work
in the past couple of years. No objections or proposed work was raised in
response to these postings. The tinyware may see a resurrection into a
separate repo for archival purposes if any users of it show interest in
doing so.

MFC after: never