docs/ntvfshandler: remove duplicate value

docs/ntvfshandler: remove duplicate value

The "posix" value of option "ntvfs handler" was written two times. This commit
deletes the first occurrence so that the default value is the first seen by
reader.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=13053

Signed-off-by: Yvan Masson <yvan@masson-informatique.fr>
Reviewed-by:  Marc Muehlfeld <mmuehlfeld@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>
Autobuild-User(master): Volker Lendecke <vl@samba.org>
Autobuild-Date(master): Wed Sep 27 18:44:45 CEST 2017 on sn-devel-144