[rfc] remove ``names.scan()``

As far as I can grep, ``names.scan()`` alias ``scan_external_dir()`` is used nowhere and undocumented. There’s no test for it either.  I’ve read some old posts on mailing lists and tex-sx that would suggest was intended to be called  from fontspec. However, fontspec seems to do fine without it, so I guess we might as well drop the function.