ocfs2: Remove unused ocfs2_restore_xattr_block().

Since now ocfs2 supports empty xattr buckets, we will never remove
the xattr index tree even if all the xattrs are removed, so this
function will never be called. So remove it.

Signed-off-by: Tao Ma <tao.ma@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>