Move natural=spring back to amenity-points layer

[jeisenbe]

Fixes #4011

Changes proposed in this pull request:

• Move natural=spring icon back to amenity-points layer, delete current springs layer
• Previously springs were rendered below water-lines. This is not longer the case, so springs can be rendered with other symbols again
• Springs now render above paths and highways, like other point symbols.
• This change should slightly improve performance, and springs will no longer render with priority to placenames such as hamlets and villages.

Test rendering with links to the example places:

z14 Before

z14 After - place label now seen (upper left)

z15 before

z15 after

z16 before

z16 after

• Shinto place of worship, place name, and funicular station now all show with priority over the springs.

[imagico]

Note this symbol for springs was never intended to be used as a POI style icon.

I would probably rather prefer to see a full revert of #3189 for the reasons explained in #325 (comment) but there is likely no consensus for that.

[jeisenbe]

I am in favor of changing the spring icon. I think @kocio-pl also thinks it is too large at low zoom level. We discussed this previously but did not reach consensus in #3461

I would support using a much smaller, abstract icon at low zoom levels and a slightly smaller icon at high zoom levels.

But this PR will be an improvement, since springs are currently blocking placenames that appear sooner.

[jeisenbe]

I also would like to see geysers and hot springs rendered: #3461 (comment) - also see test images in #3461 (comment)

[pnorman]

I'm also in favour of changing the spring icon, but this helps with the priority issues and doesn't make the icon situation worse.