Workbench: Make Material transparency part of the rgba color picker

It is only used for solid mode for now but could be used by eevee in the
future.

release/scripts/startup/bl_ui/properties_material.py

@@ -258,7 +258,6 @@ class MATERIAL_PT_viewport(MaterialButtonsPanel, Panel):
         col.prop(mat, "diffuse_color", text="Color")
         col.prop(mat, "metallic")
         col.prop(mat, "roughness")
-        col.prop(mat, "transparency")
 
     def draw(self, context):
         self.draw_shared(self, context.material)