Get rid of unused CubicCurve shaders

There are a number of CubicCurve shaders compiled as part of our build and referenced by a single method in the Graphics class that is never called (despite its name it isn't used to render actual Cubic Curve path segments and its external interface appears to involve a list of triangles so it's not clear what case this was implemented to solve - perhaps it was related to the tesselation code that was recently removed?).