Mapserverproxy layer filtering fix

As for now, when using the layer filtering tool (useful for instance to show only authorized layers in a GetCap), an intersection is done between the authorized layers list and the LAYERS param provided in the request (if provided).

In some cases (eg. with a mapserver GROUP: one might have defined the GROUP in the layer admin but the request might directly use the grouped layers => the intersection fails), this test is too restrictive and will incorrectly filter layers that should actually be available.

Solution: remove the intersection.