Remove scale.js extension

[devunwired]

By default, this backend seems to inject the deck.scale.js extension into all generated HTML. When rendering Asciidoc content with long sections (lots of text 20-30+ lines or so), this causes the slides to shrink down to a minuscule size in order to fit the window height—rather than allowing the long content to be scrollable.

I personally would not like to see this extension used, but perhaps it is better to create an Asciidoc attribute that controls how the "slide height" is handled (scroll, scale, etc.)?