Remove redundant and generic variables and mixins

[lewisnyman]

I think a general rule would be to only have mixins and variable that pertain to the WunderKraut brand, anything more generic than that should sit in a more generic extension and it probably already does.

Some mixins, like the font mixins, must be included for the purpose of designing/prototyping a new site and we don't need them here. eg:

$sans-serif-large Verdana, Geneva, 'DejaVu Sans', Arial, Helvetica, sans-serif;