SERVER-45338 added a dedicated mono-threaded executor to be used for moveChunk commands (then moved to manage moveRange commands starting from v6.0) but there seems to be no reason to keep on using a separate executor rather than picking a thread from the fixed executor.

The reason behind this comment seems to be that the idea was having a one-threaded executor to guarantee that only one migration at a time could run, however the same guarantee is already provided by the ActiveMigrationsRegistry.