Remove Browserupgrade prompt

[coliff]

I'm submitting a ...

• bug report
• feature request
• other (Please do not submit support requests here (below))

HTML5Boilerplate default HTML includes a browser upgrade prompt which only displays on IE9 and below. I suggest that this should be removed in the next version of H5BP because:

• It doesn't display on IE10 (as IE10 doesn't support IE conditional comments)
• An IE8 user seeing the 'upgrade your browser' link does not provide supported alternatives. Chrome/Firefox/Opera are no longer supported on XP or Vista which an IE8 user would have.
• In my limited testing https://browsehappy.com/ doesn't actually load on IE8 anymore (HTTPS/SSL issue maybe?)
• IE8/IE9 marketshare is tiny

Instead, it'd possibly be a better idea for developers to detect version of IE in JavaScript (possibly by detecting the version of Windows too). If user is on Windows 7 or 8 - they should update to IE11 or download Edge. I think adding info or code for this is maybe out of scope of this project though, so suggest we simply remove the browserupgrade prompt.