Merge #11638: [tests] Dead mininode code

Merge #11638: [tests] Dead mininode code

fb00c45c3 [tests] Explicitly disallow support for p2p versions below 60001 (John Newbery)
3858aabbd [tests] Remove support for p2p alert messages (John Newbery)
c0b127470 [tests] Remove support for bre-BIP31 ping messages (John Newbery)
2904e301c [tests] Remove dead code from mininode.py (John Newbery)

Pull request description:

  This is the first part of #11518. It removes a ~150 lines of unused code from the mininode module:

  - remove unused `deliver_sleep_time` and `EarlyDisconnectError` code
  - remove support for pre-BIP31 ping messages
  - remove support for alert message
  - explicitly don't support p2p versions lower than 60001

  Should be an easy ACK for reviewers. If all extended tests pass, then this code really was dead :)

Tree-SHA512: 508e612ceb0b094250d18e75522d51e6b14cd069443050ba4af34d6f890c58721cb5653e8bc000b60635b9474d035b0dcd9c509c0dcdb3a7501df17b787f83b0