DolphinQt: Remove redundant window hints

Remove window hints clearing the flag Qt::WindowContextHelpButtonHint,
which is already off by default in Qt 6.

In Qt 5 this flag was set by default for QDialogs, and on Windows put a
? button in the corner of the title bar allowing users to activate Qt's
QWhatsThis help system for a given widget. Since we don't set that text
the ? button was useless and so we hid it manually.

Source/Core/DolphinQt/AboutDialog.cpp

@@ -15,7 +15,6 @@
 AboutDialog::AboutDialog(QWidget* parent) : QDialog(parent)
 {
   setWindowTitle(tr("About Dolphin"));
-  setWindowFlags(windowFlags() & ~Qt::WindowContextHelpButtonHint);
 
   QString branch_str = QString::fromStdString(Common::GetScmBranchStr());
   const int commits_ahead = Common::GetScmCommitsAheadMaster();