Monospace code blocks by removing bold

Looking at the [registration code sample](http://kripken.github.io/emscripten-site/docs/api_reference/html5.h.html#registration-functions), I noticed that the code blocks are not monospaced, although they purport to be. This is apparent in the mismatched comments.
The reason for this is that the selected font, `Inconsolata`, has a different width when bolded than when normal.

I've fixed this by going through the `.css` for code blocks and removing all directives to bold the code.
Alternatively, this could be solved by selecting a different font. I justify choosing the former because code is normally written in colored monospace fonts, without bolding or italicizing, so this still appears natural - arguably more so.

As far as I can tell (from `ack \.\.\ code-block::`) this is only used in `api_reference/html5.h.rst` and `api_reference/preamble.js.rst`.

##### Changes:
 - Removed css to set `font-weight:bold` within the .codeblock-example class/subclasses.