dm btree remove: remove unused function get_nr_entries()

rebalance_children() calls get_nr_entries() and assigns the result to an
unused local 'child_entries' variable.  Remove get_nr_entries() and
cleanup rebalance_children() accordingly.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>