Remove border_radius support for slider::Rail

Our current quad shader may produce weird-looking results with
non-integral scaling factors.

style/src/slider.rs

@@ -17,8 +17,6 @@ pub struct Rail {
     pub colors: (Color, Color),
     /// The width of the stroke of a slider rail.
     pub width: f32,
-    /// The border radius of the slider.
-    pub border_radius: f32,
 }
 
 /// The appearance of the handle of a slider.

style/src/theme.rs

@@ -422,7 +422,6 @@ impl slider::StyleSheet for Theme {
                             palette.primary.base.color,
                         ),
                         width: 2.0,
-                        border_radius: 2.0,
                     },
                     handle: slider::Handle {
                         color: palette.background.base.color,