onboarding: Go back to not having system be separate (#35499)

Going back to having system be mutually exclusive with light/dark to
simplify the system. We instead just show both light and dark when
system is selected

Release Notes:

- N/A

---------

Co-authored-by: Danilo Leal <daniloleal09@gmail.com>
Co-authored-by: Anthony Eid <hello@anthonyeid.me>

crates/gpui/src/geometry.rs

@@ -3522,7 +3522,7 @@ impl Serialize for Length {
 /// # Returns
 ///
 /// A `DefiniteLength` representing the relative length as a fraction of the parent's size.
-pub fn relative(fraction: f32) -> DefiniteLength {
+pub const fn relative(fraction: f32) -> DefiniteLength {
     DefiniteLength::Fraction(fraction)
 }