Ref should be dropped in favor only of network and shield_text #1027
Description
Now that we're relying on network and shield_text (see #966) to do the heavy lifting instead of ref, we should probably drop ref from low- and mid-zooms (or maybe entirely) and modify merge logic to use the shield bits instead.
Probably only the singular network and shield_text at zooms 0-8 and at 9+ all_networks and all_shield_texts.
Example:
# Way: I 81 (302933871)
# http://www.openstreetmap.org/way/302933871
# Example tile: `14, 4496, 6381` in **roads** layer.