os: handle trailing slashes in os.RemoveDir on Windows
CL 661575 inadvertently caused os.RemoveDir on Windows to
fail when given a path with a trailing / or \, due to the
splitPath function not correctly stripping trailing
separators.
Fixes #73317
Change-Id: I21977b94bb08ff1e563de6f5f16a4bdf5024a15e
Reviewed-on: https://go-review.googlesource.com/c/go/+/664715
Auto-Submit: Damien Neil <dneil@google.com>
TryBot-Bypass: Dmitri Shuralyov <dmitshur@golang.org> Reviewed-by: Alan Donovan <adonovan@google.com>