[z2052] Prevent unloading go before all players left capture point zone

This fixes m_UniqueUsers not being cleared for example if grid is set to idle and would therefore think that some players are still in range on grid reactivation causing the slider not to appear

(based on commit 444f3c7)

Signed-off-by: Nighoo <nighoo@overninethousand.de>

(based on commit [s1666] - ffb9477)