Fix warning about self assign

...tst_qlineedit.cpp:1938:9: warning: explicitly assigning value of variable of type 'Qt::Key' to itself
-Wself-assign]
    key = key;
    ~~~ ^ ~~~

Change-Id: I18a46f61e13f6e2c74edce869a1c36a7f3a0fb70
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

tests/auto/widgets/widgets/qlineedit/tst_qlineedit.cpp

@@ -1937,9 +1937,8 @@ static void figureOutProperKey(Qt::Key &key, Qt::KeyboardModifiers &pressState)
         break;
     }
 #else
-    // Naively kill the warning.
-    key = key;
-    pressState = pressState;
+    Q_UNUSED(key);
+    Q_UNUSED(pressState);
 #endif
 }