jfs: remove incorrect comment in jfs_superblock

There is a comment in struct jfs_superblock that incorrectly labels
a 128-byte boundary. It has never been correct.

Shenghui Wang proposed moving it to the correct spot, before s_xlogpxd,
but at this point, I believe it is best just to remove it.

Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Reported-by: Shenghui Wang <shhuiw@foxmail.com>