UI: Theme: Remove "Active Spline" theme property

In 3D Viewport theme settings there is "Active Spline" property, which behaves weirdly.
Color of that property is multiplied to color of curve handles. Even though it says active,
it's multiplied in selected and unselected states, for all curves all the time.

That doesn't make much sense, has no real value, and ends up only causing confusion.
Having this property on anything but pure black means that whatever colors you choose
for curve handles in Preferences isn't actually what you're getting them. If color is set to
high-saturation color it completely washes away all colors and makes it difficult to differentiate
between handle types.

I think there is no reason for this property to exist, so this PR just removes that property.

Pull Request: https://projects.blender.org/blender/blender/pulls/145360

source/blender/draw/engines/overlay/overlay_instance.cc

@@ -342,7 +342,6 @@ void Resources::update_theme_settings(const DRWContext *ctx, const State &state)
   UI_GetThemeColor4fv(TH_NURB_VLINE, gb.colors.nurb_vline);
   UI_GetThemeColor4fv(TH_NURB_SEL_ULINE, gb.colors.nurb_sel_uline);
   UI_GetThemeColor4fv(TH_NURB_SEL_VLINE, gb.colors.nurb_sel_vline);
-  UI_GetThemeColor4fv(TH_ACTIVE_SPLINE, gb.colors.active_spline);
 
   UI_GetThemeColor4fv(TH_CFRAME, gb.colors.current_frame);
   UI_GetThemeColor4fv(TH_FRAME_BEFORE, gb.colors.before_frame);