xfs: remove dead stale buf unpin handling code

xfs: remove dead stale buf unpin handling code

This code goes back to a time when transaction commits wrote
directly to iclogs. The associated log items were pinned, written to
the log, and then "uncommitted" if some part of the log write had
failed. This uncommit sequence called an ->iop_unpin_remove()
handler that was eventually folded into ->iop_unpin() via the remove
parameter. The log subsystem has since changed significantly in that
transactions commit to the CIL instead of direct to iclogs, though
log items must still be aborted in the event of an eventual log I/O
error. However, the context for a log item abort is now asynchronous
from transaction commit, which means the committing transaction has
been freed by this point in time and the transaction uncommit
sequence of events is no longer relevant.

Further, since stale buffers remain locked at transaction commit
through unpin, we can be certain that the buffer is not associated
with any transaction when the unpin callback executes. Remove this
unused hunk of code and replace it with an assertion that the buffer
is disassociated from transaction context.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>