Drop get_payload_content method in Kafka bootstep

There is a FIXME comment in https://git.ligo.org/emfollow/gwcelery/-/blob/v2.5.1/gwcelery/tasks/alerts.py?ref_type=tags#L217-218:

    # FIXME Drop get_payload_content method once
    # https://github.com/scimma/hop-client/pull/190 is merged

The upstream issue has been fixed; see https://github.com/scimma/hop-client/pull/190#issuecomment-2468866071.

assigned to @cody.messick

Thanks for making this issue Leo, I'd made a mental note to do this but having the issue is very helpful. We need to wait for a new hop client release, but I'll get a MR going once that happens.