Undefined property fix

[eileenmcnaughton]

This removes interaction with an undefined property which actually only tells us if eventId is in the url. The property is only accessed in preProcess, buildForm & setDefaultValues - ie when the GET is populated because this overload use case is only for ajax loading of fees - so we can just keep checking _GET and not set a property

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[yashodha]

@eileenmcnaughton looks good, merging this PR.

[eileenmcnaughton]

thanks @yashodha