Use systemProxyForQuery() everywhere

For generic Unix, Qt automatically picks up http_proxy since
Qt 4.8 (commit 2ec929a2b). It also picks up e.g. https_proxy etc,
so it's better to rely on it.

On OS X we want to use the system proxy settings, that allow
also more advanced configuration like different proxies for
different services, passwords ...

Change-Id: I72de5e576e4d34d87f4c787bb597db0ca59a70e1
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/app/main.cpp

@@ -393,20 +393,8 @@ int main(int argc, char **argv)
     }
 
     // Make sure we honor the system's proxy settings
-#if defined(Q_OS_UNIX)
-    QUrl proxyUrl(QString::fromLatin1(qgetenv("http_proxy")));
-    if (proxyUrl.isValid()) {
-        QNetworkProxy proxy(QNetworkProxy::HttpProxy, proxyUrl.host(),
-                            proxyUrl.port(), proxyUrl.userName(), proxyUrl.password());
-        QNetworkProxy::setApplicationProxy(proxy);
-# if defined(Q_OS_MAC) // unix and mac
-    } else {
-        QNetworkProxyFactory::setUseSystemConfiguration(true);
-# endif
-    }
-#else // windows
     QNetworkProxyFactory::setUseSystemConfiguration(true);
-#endif
+
     // Load
     const QStringList pluginPaths = getPluginPaths() + customPluginPaths;
     PluginManager::setPluginPaths(pluginPaths);