Mimetypes v3: Never use installed mime data

It gets in the way, e.g. "No plugin can open project type "application/
x-kicad-project"" when trying to open .pro files. Mostly relevant on
Linux, where mime data is actually installed system-wide.

Re-implements 4f5f5d75b4

Change-Id: Ifd34214b99b77720cfc6d22e6702ca6e9ba513dc
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

src/libs/utils/mimetypes2/mimedatabase.cpp

@@ -101,8 +101,13 @@ bool MimeDatabasePrivate::shouldCheck()
 
 void MimeDatabasePrivate::loadProviders()
 {
+#if 0
     // We use QStandardPaths every time to check if new files appeared
     const QStringList mimeDirs = QStandardPaths::locateAll(QStandardPaths::GenericDataLocation, QLatin1String("mime"), QStandardPaths::LocateDirectory);
+#else
+    // Qt Creator never uses the standard paths, they can conflict with our setup
+    const QStringList mimeDirs;
+#endif
     const auto fdoIterator = std::find_if(mimeDirs.constBegin(), mimeDirs.constEnd(), [](const QString &mimeDir) -> bool {
         return QFileInfo::exists(mimeDir + QStringLiteral("/packages/freedesktop.org.xml")); }
     );