Remove duplicate CSS caused by mixins

Removed parentheses in declaration of mixins (in mixins.less) which is going to be accessible as a class.

Removed the classes (in utilities.less) which has already been declared as mixins without parentheses, as they will be added to the final CSS.

In LESS any class can be used as a mixin, but when we declare a mixin and afterwards add a class with the same name, LESS actually duplicates the mixin. When we apply e.g. the .clearfix to the .row-selector, the clearfix CSS is added twice to the final CSS.