[firefox] remove unused update dockerfiles

[jankeromnes]

@ishitatsuyuki We don't use these two files anymore, but I seem to remember that you may want to keep them, so I'm asking you for review on this. :slightly_smiling_face: Please have a look when you have time.

[ishitatsuyuki]

I don't have any objection for now. If we should implement effective incremental builds in the future, bringing this back is trivial so it's fine to me.

[jankeromnes]

Thanks! Works for me.