Comment out mapping of seq_region_id to an external_seq_region_id.

For release 64 we've found that features on human HSCHR5_1_CTG1 would
not get fetched since its seq_region_id is an internal_seq_region_id
with a corresponding, but non-existent, external_seq_region_id in the
seq_region_mapping table.

modules/Bio/EnsEMBL/DBSQL/BaseFeatureAdaptor.pm

@@ -597,7 +597,8 @@ sub _slice_fetch {
 
       # If there is mapping information, use the external_seq_region_id
       # to get features.
-      $sr_id = $self->get_seq_region_id_external($sr_id);
+      ##FIXME## $sr_id = $self->get_seq_region_id_external($sr_id);
+
       $constraint .= " AND " if($constraint);
 
       if ( !$slice->is_circular() ) {