Don't use deprecated QTextStreamFunctions::flush()

Qt doc states: "If QTextStream operates on a string, this function does
nothing". So remove it.

src/app/cmdoptions.cpp

@@ -565,7 +565,6 @@ QString makeUsage(const QString &prgName)
            << QLatin1String("QBT_NO_SPLASH=1 ") << prgName << '\n'
            << wrapText(QObject::tr("Command line parameters take precedence over environment variables"), 0) << '\n';
 
-    stream << flush;
     return text;
 }
 

src/gui/search/searchwidget.cpp

@@ -99,7 +99,7 @@ SearchWidget::SearchWidget(MainWindow *mainWindow)
            << tr("<b>&quot;foo bar&quot;</b>: search for <b>foo bar</b>",
                  "Search phrase example, illustrates quotes usage, double quoted"
                  "pair of space delimited words, the whole pair is highlighted")
-           << "</p></body></html>" << flush;
+           << "</p></body></html>";
     m_ui->lineEditSearchPattern->setToolTip(searchPatternHint);
 
 #ifndef Q_OS_MACOS