Remove / Refactor the Rel-mixin

Based on the reviewer feedback on !1362 (comment 343088394),

question: Why did we choose to 'keep' href inside attrs and not promote it as a prop since we are actively using it in the component? That way we can have type checking and clearly state that the href is part of this component interface ( we could also get rid of inheritAttrs: false )

We should either remove or refactor the rel_mixin.

Note - The mixin is used by GlButton.