Remove PartialOrd implementation for Rectangle

A `PartialOrd` implementation is unclear for this type, since it has
a position besides its dimensions.

core/src/rectangle.rs

@@ -111,12 +111,6 @@ impl Rectangle<f32> {
     }
 }
 
-impl std::cmp::PartialOrd for Rectangle<f32> {
-    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
-        (self.width * self.height).partial_cmp(&(other.width * other.height))
-    }
-}
-
 impl std::ops::Mul<f32> for Rectangle<f32> {
     type Output = Self;