Eliminate `contributeMode`

contributeMode is the old concept when we thought we could categorise payment processors into 3-4 basic types that did the same stuff. We have mostly eliminated but it perseveres in a few places we want to kill

Form	Used for	Alternative
Contribution_Confirm	Should the form postProcess hook be called before doPayment	? $processor->supports('formAssumesNoReturn') (I kinda feel it should be on the processor to call the hook but...
Contribution_Confirm	Should the button be called 'Continue' or 'Make Contribution'	Ditch? always 'Make Contribution'? Or do we need a getText('continue_contribution_button)
Event_Registration	Should emails be sent from the form (otherwise sent on completetransaction)	? $processor->supports('formAssumesNoReturn')
Event_Registration_Confirm	Should paypal express code be called	probably just a different hack
Event_Registration_Confirm::554	several places tricky stuff around doPayment	will need unravelling
Contribution Confirm.tpl	What text should we use for continue_instructions-section	?paymentObject->getText('continue_contribution_instruction') ?
Registration Thankyou.tpl	What text should be used arount 'Your registration has been processed successfully'	?paymentObject->getText('event_success_instruction') ?

All other places are either doing nothing or are removable