Fix rupture distance filtering in Point and Area Source

This pr fix the way rupture-site filtering is done for Point and Area sources.

Currently filtering is done based on epicentral distance, while it should be done in terms of Joyner-Boore distance to be consistent to what is done for fault sources (see https://bugs.launchpad.net/oq-hazardlib/+bug/1241432)

This pr simply removes the 'filter_sites_by_distance_to_rupture' method implemented in the ``PointSource`` class which is also used by the ``AreaSource`` class, so that these two classes make use of the implementation done in the base class which uses the Joyner-Boore distance