Consider removing Grunticon's symbols.fallback.css method

[jonnystening]

This method is used by browsers that don't support data URIs, such as IE6 & IE7 http://caniuse.com/#search=data%20uri (click Show all).

If we're no longer officially supporting these browsers (likely) the Grunticon output (/img/*.png files and symbols.fallback.css file are no longer required. Less for git, the Blender and Developer to have to worry about.

NB. the symbols.data.png.css file is used by IE8 (which doesn't support SVGs). The above could be considered in this case too, as IE8 is likely not a concern these days. http://caniuse.com/#feat=svg-css (click Show all)