CRM-2950 conditional display of Fee Levels block in event fees configuration is not correct when page reloaded

The "Fee Level" block on the "Event Fees" configuration page is hidden when a Price Set is enabled, however if the page reloads due to a form validation error and a price set is selected, then the Fee Level block is shown.

To recreate:
1. Go to configuration page of an event and select "Event Fees"
2. Set "Paid Event=Yes", and select a Price Set in the drop-down. The "Fee Level" block dissappears.
3. CLEAR the "Fee Label" field.
4. Hit Save button.
5. Observe that the page reloads with an error because Fee Label was blank, but also shows that the Price Set is selected AND the Fee Level block is shown.

Correct behaviour is for Fee Level block to be conditionally hidden even when reloading after a validation error.