sys/dev/disk/dm: Remove upcall handler

sys/dev/disk/dm: Remove upcall handler

dm target's upcall() handler
  int (*upcall)(dm_table_entry_t *, struct buf *);
implemented by dm targets acutally do nothing other than
returning 0. Also note that upcall() is not used by dm core.

The targets that are supposed to be relying on this api are
obviously not working at the moment, however things aren't
as simple as just implementing missing upcall() handler.

upcall() is supposed to be something to do with targets like
snapshot, but it lacks documentation and purpose of this api
is not clear at all. Whoever tries to implement snapshot/etc
will have to re-design dm core and appropriate handlers from
scratch anyway without using the existing one.