Revert CL 137055, which changed Clean("\\somepath\dir\") to return
"\\somepath\dir" on Windows. It's not entirely clear this is correct,
as this path is really "\\server\share\", and as such the trailing
slash may be the path on that share, much like "C:\". In any case, the
change broke existing code, so roll it back for now and rethink for 1.13.