Get rid of DisableCustomScaling hack

This shouldn't be needed since Qt is built without High-DPI scaling support in official binaries and QT_SCALE_FACTOR is a debug environment variable, so any use of it is a user's environment misconfiguration.

https://doc.qt.io/qt-6/highdpi.html#environment-variable-reference

Telegram/SourceFiles/core/sandbox.cpp

@@ -366,7 +366,6 @@ void Sandbox::singleInstanceChecked() {
 		LOG(("App Info: Detected another instance"));
 	}
 
-	Ui::DisableCustomScaling();
 	refreshGlobalProxy();
 	if (!Logs::started() || !Logs::instanceChecked()) {
 		new NotStartedWindow();