Paid Event can be configured without a Payment Processor for sites using offline registration only

Configure Event >> Event Fees form should allow an admin to save fees for a paid event WITHOUT setting up and selecting a payment processor. This use case if for events where all registration is done offline (e.g. by an organization staff member or volunteer using the contact view >> events tab).

NOTE: If a paid event does NOT have an associated payment processor - and a user goes to the online registration page (event/register) - we throw a fatal error. This behavior is unchanged from 1.9.