xfs: remove unused delta tracking code in xfs_bmapi

xfs: remove unused delta tracking code in xfs_bmapi

This code was introduced four years ago in commit
7a5e2e47995fbacc232998f08137daa4aa03e6af without any review and has
been unused since.  Remove it just as the rest of the code introduced
in that commit to reduce that stack usage and complexity in this central
piece of code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>