Make CA/FT/LMS respect g_use_ammunition

Makes it possible to play CA/FT/LMS with unlimited ammo.

Right now g_weaponarena and thus CA/FT/LMS ignore g_use_ammunition. Because these gamemodes enable g_weaponarena, which in turn forcefully enables unlimited ammo, these modes forcefully disable it back. So in practice the cvar gets ignored and it's not possible to play them with unlimited ammo. This change makes it so it's possible.

Another proposed fix for me would be for g_weaponarena to respect g_use_ammunition as well, or have separate cvars (g_weaponarena_use_ammunition, g_ca_use_ammunition, g_ft_use_ammunition, g_lms_use_ammunition) but maybe it's too much. This is the simplest workaround for now.