Bump Qt minimum version to 5.9 (#7204)

* bump qt minimum version to 5.9

* cleanup cmake checks

* Remove the obsoleted check.

* Missed this while removing the check.

src/core/ConfigManager.cpp

@@ -192,9 +192,7 @@ QStringList ConfigManager::availableVstEmbedMethods()
 {
 	QStringList methods;
 	methods.append("none");
-#if QT_VERSION >= 0x050100
 	methods.append("qt");
-#endif
 #ifdef LMMS_BUILD_WIN32
 	methods.append("win32");
 #endif

src/core/main.cpp

@@ -356,9 +356,7 @@ int main( int argc, char * * argv )
 	// don't let OS steal the menu bar. FIXME: only effective on Qt4
 	QCoreApplication::setAttribute( Qt::AA_DontUseNativeMenuBar );
 #endif
-#if QT_VERSION >= QT_VERSION_CHECK(5, 6, 0)
 	QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
-#endif
 	QCoreApplication * app = coreOnly ?
 			new QCoreApplication( argc, argv ) :
 					new gui::MainApplication(argc, argv);

src/gui/MainWindow.cpp

@@ -324,10 +324,7 @@ void MainWindow::finalize()
 					SLOT(onExportProjectMidi()),
 					Qt::CTRL + Qt::Key_M );
 
-// Prevent dangling separator at end of menu per https://bugreports.qt.io/browse/QTBUG-40071
-#if !(defined(LMMS_BUILD_APPLE) && (QT_VERSION < 0x050600))
 	project_menu->addSeparator();
-#endif
 	project_menu->addAction( embed::getIconPixmap( "exit" ), tr( "&Quit" ),
 					qApp, SLOT(closeAllWindows()),
 					Qt::CTRL + Qt::Key_Q );
@@ -400,10 +397,7 @@ void MainWindow::finalize()
 							this, SLOT(help()));
 	}
 
-// Prevent dangling separator at end of menu per https://bugreports.qt.io/browse/QTBUG-40071
-#if !(defined(LMMS_BUILD_APPLE) && (QT_VERSION < 0x050600))
 	help_menu->addSeparator();
-#endif
 	help_menu->addAction( embed::getIconPixmap( "icon_small" ), tr( "About" ),
 				  this, SLOT(aboutLMMS()));