Refactor: remove game::get_v_player, use game::get_players

[Richèl Bilderbeek]

This is a follow-up from #662.

Currently, the game has a member function called get_v_player that returns the players, where v stands for vector.
However, NL.5: Avoid encoding type information in names, hence this should be renamed to get_players.

Luckily, in #662 we just added that function name.

In this Issue, rename all calls to get_v_player to get_players. Delete the test written in #662, as it tests nothing anymore.