UI: Remove paint curve theme colors, use gizmo colors instead

3D Viewport and Image Editor had theme colors for paint curve handle and pivot.
Paint curves are one of the stroke methods for brushes, and are generally not
widely used. To simplify theming it's best to remove those four properties and
use other shared colors instead.

We could use common curve colors for them, but they don't have point color
and handle point color is black, so we would need to add one more property
anyway and it would look bad as well.

Instead, use gizmo primary and secondary colors. Technically, paint curves
can be treated as gizmos, because they're interactive widgets in the viewport.

Pull Request: https://projects.blender.org/blender/blender/pulls/146777

scripts/presets/interface_theme/Blender_Light.xml

@@ -527,8 +527,6 @@
         skin_root="#b44d4d"
         view_overlay="#000000"
         transform="#ffffff"
-        paint_curve_handle="#7fff7f7f"
-        paint_curve_pivot="#ff7f7f7f"
         outline_width="1"
         object_origin_size="6"
         >
@@ -674,8 +672,6 @@
         uv_shadow="#707070ff"
         metadatabg="#000000"
         metadatatext="#ffffff"
-        paint_curve_handle="#7fff7f7f"
-        paint_curve_pivot="#ff7f7f7f"
         >
         <space>
           <ThemeSpaceGeneric