Fix issues with Cheering and PushOrderFront

Make sure that any pending cheering order is deleted when leaving the Cheering state, otherwise other unitAI states can behave weirdly.

This fixes issues with units cheering at incorrect time, then not correctly picking their next target in a fight.

Reported by: snelius

Reviewed By: Angen

Refs #5966

Differential Revision: https://code.wildfiregames.com/D3452