use gtk3 vclplug by default under GNOME3 if available

Change-Id: I4efe8bdfb7080365094306aee9db6b69a7f9e86a
diff --git a/vcl/unx/generic/plugadapt/salplug.cxx b/vcl/unx/generic/plugadapt/salplug.cxx
index d49dccd..0d0b26d 100644
--- a/vcl/unx/generic/plugadapt/salplug.cxx
+++ b/vcl/unx/generic/plugadapt/salplug.cxx
@@ -68,14 +68,6 @@ static SalInstance* tryInstance( const OUString& rModuleBase, bool bForce = fals
        return svp_create_SalInstance();

    SalInstance* pInst = NULL;
    // Disable gtk3 plugin for now unless explicitly requested via
    // SAL_USE_VCLPLUGIN=gtk3 (would ideally depend on experimental mode, but
    // reading the experimental mode setting requires the UNO service manager
    // which has not yet been instantiated):
    if (!bForce && rModuleBase == "gtk3")
    {
        return NULL;
    }
    OUString aModule(
#ifdef SAL_DLLPREFIX
            SAL_DLLPREFIX