Remove ref_* Attributes

[Aniak5]

@rroupski and I discussed removing all the original event source attributes (ref_* as seen below) and allowing them to be specified in the unmapped base event attribute. This would simplify the question of where event source specific data should go as well as reduce the number of source specific attributes.

[jp-harvey]

I'm voting to remove them but wanted to add some additional context: ref_time is probably important enough to be retained but named to something like original_time and be changed to optional.

[oshezaf]

I find standardizing the naming for the original fields useful, espeically for a human user.