Removed comments before HTML doctype for IE8 standards mode

[Persona says](https://developer.mozilla.org/docs/persona/Browser_compatibility#Internet_Explorer_.22Compatibility_Mode.22):

> Because versions of Internet Explorer earlier than 8.0 are not supported by Persona, any version of Internet Explorer which is configured to emulate a pre-8.0 version will also not function with Persona. This is typically for one of the following reasons:
> 
>  * your site is using "X-UA-Compatible" to explicitly instruct the browser to emulate a pre-8.0 version
>  * your site's pages omit the DOCTYPE, do not have the DOCTYPE as the first line of the page, or set the browser to quirks mode, and your site is not setting "X-UA-Compatible" to IE version 8.0 or higher
>  * the browser is locally configured to use a pre-8.0 Compatibility Mode, and your site is not overriding this by setting "X-UA-Compatible" to IE version 8.0 or higher