Remove support for tunnel=true and tunnel=1

At the moment there is code tucked away in project.mnl to convert tag values true and 1 to a synthetic tag yes when the key is tunnel (also bridge and oneway but not others such as ref). Using synthetic tags instead of using the style sheet lacks transparency.

CARTOGRAPHY.md states:
>: While this desire is in obvious conflict with the above comments on level of detail, it has a second impact on the map style. We need to, wherever possible, avoid accidentally encouraging mistakes - that is, avoiding the situation where a clearly misspelled or misused tag leads to the originally expected result. So highway=mtorway shouldn't show up as if it was highway=motorway, and so on. This has been a problem with "catch-all" rules in queries and filters, such as where leisure is not null or [amenity != ""]. Avoid these situations.


  According to http://taginfo.openstreetmap.org/keys/tunnel#values the tag tunnel=true is entirely missing from the database and there are 12 occurrences of tunnel=1, tunnel=undefined and tunnel= (empty string) are commoner. I am unaware any active arguments to prefer these tags that removing support would interfere with.