Drop lobby presence GUI messages altogether for better performance and less code complexity.
They were only used to determine whether the playerlist should be rebuilt, which can be achieved by a boolean member.
So this patch is removing unnecessary indirection from the original solution in rP16997 and the reduction in rP20040, refs #3386.
In particular the LobbyClearPresenceUpdates call was ugly and one doesnt need hundreds of messages with data on it each if one only wants to know if there was one.
Makes past (rP20070) and future (D2264) patches less complex, refs #4877.
Differential Revision: https://code.wildfiregames.com/D2265
Tested on: gcc 9.1.0, clang 8.0.1, Jenkins