Prior to #7177, these benchmarks did not provide any useful data about the
performance of the pubsub system (in fact, prior to #7178, half of them did not
work at all).
Specifically, they create a bunch of subscribers with 1 buffer slot on a
default publisher and copy messages to them. But because the publisher is
single-threaded, and doesn't block for delivery, all this tested is how long it
takes to receive a single message from a channel and deliver it to another
channel. The resulting stat does not even vary meaningfully with batch size,
since it's testing a serial workload.
Since #7177 the benchmarks do correctly reflect delivery time (good), but still
do not tell us anything useful: The latencies that matter for pubsub are not
internal queuing, but the effects of backpressure on the publisher via the
subscribers. That's an integration problem, and simulating a fake workload does
not provide meaningful results.
On that basis, remove these benchmarks.