Remove default_symbol from the VectorTileStyle

[Maximkaaa]

The default symbol structure is very strange, as it is a collection of symbols of different types. It is especially wrong when it comes to point symbol and label symbol, because both of them are used to render point geometries.

But as it turns out we don't need this structure at all. It can be replaced by the style rules with no filters, which is much more flexible. We can also introduce logic in the vector tile processor to apply a styling rule to a feature only if the symbol type and layer geometry type are aligned. This will allow us to have three different style rules with no filters, each of which will apply to the corresponding geometry type.

Doing this requires to:

• remove the field from the style structure
• remove the logic in the vt_processor that uses default symbols
• change the styles used in the examples to reflect the change