Destroy Hyperspace clouds after 2 days of ingame time.

Hyperspace clouds continued to exist forever, now we destroy them after 2 days.
This should not be necessary. The list of hyperspace clouds in `Game` is only used while the player is in hyperspace, it's just a temporary container to allow clouds to be brought over from the source system to the target system. At all other times, `Game::m_hyperspaceClouds` should be empty.