Clean up code (#3431)

* Empty destructors are replaced with default destructors
* A few loop variables made into const references to avoid copies
* Add missing `override` spec for some `Command::execute` methods

src/cli/Clip.cpp

@@ -43,10 +43,6 @@ Clip::Clip()
         {QString("timeout"), QObject::tr("Timeout in seconds before clearing the clipboard."), QString("[timeout]")});
 }
 
-Clip::~Clip()
-{
-}
-
 int Clip::executeWithDatabase(QSharedPointer<Database> database, QSharedPointer<QCommandLineParser> parser)
 {
     const QStringList args = parser->positionalArguments();