[release-branch.go1.12] path/filepath: revert "fix Windows-specific Clean bug"

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.

Updates #27791
Updates #30307

Change-Id: I69200b1efe38bdb6d452b744582a2bfbb3acbcec
Reviewed-on: https://go-review.googlesource.com/c/163077


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
(cherry picked from commit 153c0da8)
Reviewed-on: https://go-review.googlesource.com/c/163078
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>