Remove cartesian shorthand

This removes the .x, .y, and .z properties from any object that subclasses from `SphericalCoordinatesBase` (fixes #1550). Instead, you have to do, e.g., `icrs.cartesian.x`. This is to make it more consistent with accessing representations in other coordinate systems, e.g. `icrs.galactic.l`.

cc @demitri @eteq