os: strip trailing slashes in RemoveAll on Plan 9

CL 797880 changed Root to handle trailing slashes in
paths and added TestRootConsistencyRemoveAll.

However, this test is failing on Plan 9 because the "noat"
implementation of RemoveAll disagrees with Root.RemoveAll
when a path ends in a slash and its target is not a
directory. Root.RemoveAll strips the trailing slash and
removes the file, while os.RemoveAll leaves the slash in
place. On Plan 9, Remove and Lstat then fail on the
slash-terminated path, so os.RemoveAll returns an error
instead.

This change strips trailing slashes in the "noat"
implementation of RemoveAll, so RemoveAll("not_a_directory/")
removes the file and returns nil, and remains consistent
with the "at" implementation and Root.RemoveAll.

Fixes #80319.

Change-Id: I5c47ccd239c629da78772f39a831cccf8c9d1500
Reviewed-on: https://go-review.googlesource.com/c/go/+/798721
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Richard Miller <millerresearch@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>