The callbacks aren't used in cluster mode, and puma prints a warning
about that. This is fine, we don't need the callbacks in single-mode,
but can still keep it, in case somebody switches on cluster mode.
The problem is, the current puma version has a bug, where pumactl
crashes when trying to print these warnings, so lets just silence the
warnings. People running in single mode also don't need to care about
the warnings anyway.
