Remove stale XXX comment in logical launcher

Hi,

I noticed one comment in the launcher that can be removed:

if (!retain_dead_tuples)
/* XXX unclear why we don't request logical decoding disable */
ReplicationSlotDropAcquired(false);

Since the conflict detection slot is a physical one, there is no logical
decoding state to disable when dropping it. The condition it depends on is also
self-evident from the surrounding code and the block comment above, which
already explains why the slot is dropped. So here is a patch to remove the XXX
comment.

Best Regards,
Zhijie Hou