refac: ng-model is now required
refac: ng-model is now required

There is no real purpose in using select2 without an `ng-model` so I'm simply going to make it required and remove the conditional code.
mufl1h/ui-select2,Abbaty/ui-select2,Intilery/ui-select2,brettshollenberger/ui-select,deepuke/ui-select2,angular-ui/ui-select2,mufl1h/ui-select2,deepuke/ui-select2