Clean up #roads-low-zoom layer

Changes:
- Move preserved railways and monorails from low-zoom layer #roads-low-zoom to high-zoom layer #roads-fill
- Remove funicular selector that never gets hit

This pull request solves the following problem:
- #roads-low-zoom contains also high-zoom objects

This pull request changes the rendering as follows:
- The purpose of this pull request is improving the readability of the code. However, this pull request also has minor effects on rendering order. In particular, monorails and preserved railways are now (just like other railways) displayed below aerialways, ferry routes, and the fill of turning circles, instead of above them. Depending on the z-order, monorails and preserved railways might now also be displayed below other types of roads and railways, instead of above them.