Refactor: scheduler: remove dead code

sort_instance_by_colocation() creates hash tables with one node each,
and then previously sorted the tables (which is meaningless since they only
have one node) and compared every node in them (which is redundant because the
only nodes have already been compared).

This appears to have been the case since the code was introduced in 5e7dc9f7,
apparently as a copy-paste error. All regression tests still pass, including
bug-lf-2160 added for that commit.