Always treat highway=busway as access=no

While it is technically possible for `highway=busway` to be tagged with
permissive general access values (e.g., `access=yes`), this style
chooses a strict interpretation of the tag's documentation and proposal,
and dissallows these, defaulting to no access regardless of the tags.

project.mml

@@ -726,8 +726,9 @@ Layer:
                 END AS int_surface,
                 CASE WHEN access IN ('destination') THEN 'destination'::text
                   WHEN access IN ('no', 'private') THEN 'no'::text
-                  -- highway=busway is access=no/bus=designated by default.
-                  WHEN highway IN ('busway') AND access IS NULL THEN 'no'::text
+                  -- This style assumes that highway=busway will never permit anything other than access=no, etc.
+                  -- Therefore, permissive general access values are explicitly ignored.
+                  WHEN highway IN ('busway') THEN 'no'::text
                 END AS access,
                 construction,
                 CASE