Remove Denmark primary route shield

[1ec5]

This style currently includes a yellow shield for network=dk:national under the assumption that every route in this network is a primary route. In fact, this network tag is being used for a mix of primary routes (primærrute, yellow shield) and secondary routes (sekundærrute, white shield). These are two distinct networks in reality and in signage. The currently implemented shield is premature; it should be removed so that the local community can come to a consensus about a less ambiguous tagging scheme without undue influence from this style.

/ref #142