remove blanket impl From<IntoGeometry> for GeometryCollection

[michaelkirk]

This implementation is only marginally useful, and frequently collides with other use cases.

For one, we can't implement From<GeometryCollection> for Geometry

It's a minor annoyance, not a huge problem, and it's a breaking change on geo-types, so it probably shouldn't happen until something more motivating inspires us to break geo-types.