xilem_web: Remove unused direct dep on kurbo.

[waywardmonkeys]

This is always used via peniko.

[waywardmonkeys]

This is an offshoot from #223.

[waywardmonkeys]

All uses of kurbo within xilem_web, including the svgtoy sample are via peniko.

xilem_web::svg exports both peniko and peniko::kurbo, so the svgtoy sample is a bit textually unclear about the provenance, but it is via peniko.

[DJMcNab]

Sure

I now think we probably should do this all up the stack, although I still find the tradeoffs annoying