Remove use of the unused subtypes task_type_bpart_swallow and task_subtype_multipole

No tasks are constructed using these subtypes in engine_maketasks, so they cannot be used.

added cleanup label

requested review from @pdraper

assigned to @matthieu

@matthieu I was looking at an issue reported against multipoles (in my RDMA code) and realised that task_subtype_multipole is never used in engine_maketasks. Also found a similar story for task_subtype_bpart_swallow.

Guess multipoles are only exchanged at rebuild and bparts are only ever merged now?

Thanks. These are indeed old relics of a time where the gravity had these communications.

merged

mentioned in commit f745f98c