Update "fire a simple event"

[chrisn]

The EME spec refers to "fire a simple event" in a couple of places. This was in older versions of HTML, but isn't in the current DOM spec, so EME needs updating.

[chrisn]

Having reviewed the DOM spec, we can just replace with "fire an event", as the defaults make this equivalent to "fire a simple event". Will apply this as part of #519.