Remove Default impl for CubicCurve

[BD103]

Objective

• Implementing Default for CubicCurve does not make sense because it cannot be mutated after creation.
• Closes CubicCurve::default().position() causes panic: "attempt to subtract with overflow" - Default should work for this. #11209.
• Alternative to Ensure CubicCurves with the default value have at least one CubicSegment #11211.

Solution

• Remove Default from CubicCurve's derive statement.

Based off of @mockersf comment (#11211 (comment)):

CubicCurve can't be updated once created... I would prefer to remove the Default impl as it doesn't make sense

---

Changelog

• Removed the Default implementation for CubicCurve.

Migration Guide

• Remove CubicCurve from any structs that implement Default.
• Wrap CubicCurve in a new type and provide your own default.

#[derive(Deref)]
struct MyCubicCurve<P: Point>(pub CubicCurve<P>);

impl Default for MyCubicCurve<Vec2> {
    fn default() -> Self {
        let points = [[
            vec2(-1.0, -20.0),
            vec2(3.0, 2.0),
            vec2(5.0, 3.0),
            vec2(9.0, 8.0),
        ]];

        Self(CubicBezier::new(points).to_curve())
    }
}