symbols: clean up simple shape PNGs

Simple shape PNGs (circle, square) are removed and either represented by CartoCSS internal marker types or replaced by SVG. This improves maintainability and makes e.g. color changes easier. There are no visual changes, hence no preview.

Following symbols are affected:
* bollard, block: represented by internal marker type
* bus stop: use square SVG
* (railway, tram, aerialway) station/halt: use square SVG
* waterway lock gate: represented by internal marker type
* power pole: use square SVG
* generic shop: represented by internal marker type
* tree: represented by internal marker type