Not need to clear sort flag on reception fo particles before density. This done in the cell_unskip_tasks() routine.

runner_do_sort now only looks at the do_sort and do_sub_sort flags set in the cell, not quite sure if we still need the cleanup flag.

re-factor the sorts activation into a single function to avoid repetition. turns out this is where the drift-before-sort bug was hiding, i.e. the missing drift activation in the for-loop.