Remove observer on _captureOutputPhoto during dealloc

I got a bug report that looks like the missing removal here leads to a possible exception during camera destruction.  Here's what I saw:

    An instance 0x17003b5e0 of class AVCaptureStillImageOutput was deallocated while key value observers were still registered with it. Current observation info: <NSKeyValueObservationInfo 0x178a48e50> ( <NSKeyValueObservance 0x1700d46d0: Observer: 0x12655a010, Key path: capturingStillImage, Options: <New: YES, Old: NO, Prior: NO> Context: 0x100412bd0, Property: 0x1702503e0>