When removeShard is called the balancer starts migrating all the data for sharded collections out of that shard. The chunks are migrated through the moveRange command and  when migrations complete the range deleter kick in on the shard being removed to delete the donated ranges.
The range deletions procedure can consume a non-negligible amount of resource on the node and it can affect significantly the throughput of concurrent outgoing migrations.

Considering that the shard being removed will be discarded as soon as the draining process complete, there is no reason to actually perform range deletions and garbage collect the orphaned ranges.