Don't warn about pure Python socket when not using distributed mode

[hukka]

Currently the rpc package throws a warning always, so simply importing locust results in spurious output on command line. This is a bit annoying when creating CLI tools, when in fact there's no danger of degraded performance (that is, locust is only run locally).

[justiniso]

This seems like an old issue as I'm no longer able to reproduce this. If that's not the case, can you paste the output you're seeing?