contentscript-end.js: remove overhead of short-lived function expression

[gorhill]

We can get rid of the closure, hence the overhead of the function expression created for each image/iframe/object's load/error event.

[chrisaljoudi]

@gorhill line no.?

[gorhill]

Core code.

[chrisaljoudi]

@gorhill sure, was just curious — sorry. :)

[gorhill]

The onAnswerReceived function was created for each load or error event.

I was laconic because I was about to commit the changes.