Fix resourceGatherer value modifications following rP22767

rP22767 expects player-wide modifications to invalidate entity caches.
However, this requires the entity to have an owner when calling ApplyModifiers.

ResourceGatherer caches its gather rates at Init, when entities do now yet have their correct owner, and thus this fails.
Since OnOwnershipChanged also caches gather rates however, this call is redundant (and needlessly slow) so it can just be deleted.

Differential Revision: https://code.wildfiregames.com/D2217