commented out a few lines of unused code

the Firefox add-on validation tool was raising warnings caused by the library "highlight.js".
all warnings pertained to its usage of the (unsafe) "innerHTML" attribute of DOM Elements.

all instances of "innerHTML" occur within the method: "highlightBlock".
this method is just a helper utility.
* accepts a DOM Element
* inspects the css class of both the Element as well as its parent,
  to determine whether the name of the language has been specified
* extracts text (ie: source code) from inside of the Element
* filters this text through the work-horse method: "highlight"
  - input:  name of language, string of text [raw source code]
  - output: string of text [html formatted source code]
  - notes:  all work is done using regex.
            the DOM isn't touched; it's straight text manipulation.
* updates Element.innerHTML

relevance to add-on:
* "highlight" is called directly
* all DOM operations are done directly by the add-on

changes required to bring add-on into compliance with the Firefox validation tool:
* remove the method "highlightBlock";
  it's unused/unreachable code.

summary of changes made:
* stubbed the following methods with empty functions:
  - "highlightBlock"
  - "initHighlighting"
  - "initHighlightingOnLoad"