[PlasmaComponents3] very small refactor of ToolButton shadow code

This introduces a bit more consistency to this big `if` chain.

src/declarativeimports/plasmacomponents3/ToolButton.qml

@@ -119,8 +119,10 @@ T.ToolButton {
                     return "hover"
                 } else if (keyboardFocus) {
                     return "focus"
+                } else if (control.flat) {
+                    return "hidden"
                 } else {
-                    return control.flat ? "hidden" : "shadow"
+                    return "shadow"
                 }
             }
         }