Remove simple_httpd

simple_httpd was granted a reprieve from the picobsd removal based on having
some reported user; it turns out this user isn't actually using the version
in base and merging their changes would be difficult at this point, so the
version in base will simply continue to rot. Retire it now, it may make a
comeback to ports with the improved version.

No notice issued because its current visibility has only been for ~3
months, and a notice has been previously issued about picobsd removal.