JFS: Code cleanup - getting rid of never-used debug code

I'm finally getting around to cleaning out debug code that I've never used.
There has always been code ifdef'ed out by _JFS_DEBUG_DMAP, _JFS_DEBUG_IMAP,
_JFS_DEBUG_DTREE, and _JFS_DEBUG_XTREE, which I have personally never used,
and I doubt that anyone has since the design stage back in OS/2.  There is
also a function, xtGather, that has never been used, and I don't know why it
was ever there.

Signed-off-by: Dave Kleikamp <shaggy@austin.ibm.com>