Cleanup: UI: Remove unnecessary button arguments

Passing the current enum value as a float to the "min" argument
doesn't seem to do anything. It's also the only place that the min
and max arguments to `uiDefIconTextBut` are used.

source/blender/editors/interface/interface.cc

@@ -4691,7 +4691,7 @@ static void ui_def_but_rna__menu(bContext *C, uiLayout *layout, void *but_p)
                                      UI_UNIT_X * 5,
                                      UI_UNIT_Y,
                                      &handle->retvalue,
-                                     item->value,
+                                     0.0f,
                                      0.0,
                                      description_static);
       }

source/blender/editors/space_image/image_buttons.cc

@@ -111,7 +111,7 @@ static void ui_imageuser_slot_menu(bContext *C, uiLayout *layout, void *image_p)
                       UI_UNIT_X * 5,
                       UI_UNIT_X,
                       &image->render_slot,
-                      float(slot_id),
+                      0.0f,
                       0.0,
                       "");
   }