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
3e57ecf640428c01ba1ed8c8fc538447ada1715b 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>