binderfs: remove wrong kern_mount() call

The binderfs filesystem never needs to be mounted by the kernel itself.
This is conceptually wrong and should never have been done in the first
place.

Fixes: 3ad20fe3 ("binder: implement binderfs")
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>