Simplify 5cp logic

5cp maps, which remain highly experimental for Arena: Respawn, no
longer choose any point besides the true midpoint of the map.
Backcapping to win has been kept in, but play has been reduced to only
three points (mid and both teams' second points).

tf_arena_max_streak is no longer modified, and scrambles will take
place according to server-configured Arena scramble logic.