UPSTREAM: binderfs: remove outdated comment

The comment stems from an early version of that patchset and is just
confusing now.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Christian Brauner <christian@brauner.io>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Bug: 136497735
(cherry picked from commit 7c4d08fc4d5aca073bd4ebecbb9eda5e4d858b71)
Change-Id: I8d5376e217763d7a6203a54516d4220ccdbe268d
Signed-off-by: Hridya Valsaraju <hridya@google.com>