GPU: remove 'GPU_SHADER_2D_FLAT_COLOR'

The only real difference between `GPU_SHADER_2D_FLAT_COLOR` and
`GPU_SHADER_3D_FLAT_COLOR` is that in the vertex shader the 2D
version uses `vec4(pos, 0.0, 1.0)` and the 3D version uses
`vec4(pos, 1.0)`.

But VBOs with 2D attributes work perfectly in shaders that use 3D
attributes. Components not specified are filled with components from
`vec4(0.0, 0.0, 0.0, 1.0)`.

So there is no real benefit to having two different shader versions.

This will simplify porting shaders to python as it will not be
necessary to use a 3D and a 2D version of the shaders.

In python the new name for '2D_FLAT_COLOR'' and '3D_FLAT_COLOR'
is 'FLAT_COLOR', but the old names still work for backward
compatibility.

source/blender/editors/interface/interface_panel.cc

@@ -1160,7 +1160,7 @@ static void panel_draw_aligned_widgets(const uiStyle *style,
 
     GPUBatch *batch = GPU_batch_preset_panel_drag_widget(
         U.pixelsize, color_high, color_dark, drag_widget_size);
-    GPU_batch_program_set_builtin(batch, GPU_SHADER_2D_FLAT_COLOR);
+    GPU_batch_program_set_builtin(batch, GPU_SHADER_3D_FLAT_COLOR);
     GPU_batch_draw(batch);
     GPU_matrix_pop();
   }