Send only one game register message on game setup instead of three

[Dunedan]

Relates to #3186, but I believe it's a slightly different issue.

When setting up a new game, 0ad will always send three game register messages. The only difference between those messages is:

1. the list of players is empty
2. the list of players contains the hosting player as observer
3. the list of players contains the hosting player as actual player

As that happens always it doesn't make sense to have the first two messages. Removing them would reduce the number of game register messages to process and broadcast for XpartaMuPP to 1/3 of the current messages, thus providing a nice low-hanging performance improvement.