Do not listen to old UDP prot_listener

[davidBar-On]

• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies:
  master

• Issues fixed (if any): none

• Brief description of code changes (suitable for use as a commit message):

Clear old UDP prot_listener from read_set, as currently the server's select() always return when data (that is relevant only to the worker thread) is received. This makes the main thread consume a lot of unnecessary CPU, which reduces the total throughput - especially when only one stream is used.

In my single machine environment, with the this fix the throughput is increased by about 40% for one stream test.

[bmah888]

Thanks for the pull request! This change looks good, approved. I had mixed results trying to reproduce the performance improvements you saw, but I think this PR makes the functionality of the program more correct regardless.