ovelray: drop explicit use of OVERLAY_MOUNT_OPTIONS

ovelray: drop explicit use of OVERLAY_MOUNT_OPTIONS

Tests don't need to specify $OVERLAY_MOUNT_OPTIONS for overlay mount
helpers. These options have already been assigned to MOUNT_OPTIONS
by _overlay_config_override or by _mount_opts and will be added to
mount command by _common_dev_mount_options in _overlay_mount_dirs.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>