Remove :distance special sort

This is no longer the way to order by distance in geo searches; should
fail fast.

[sunspot#85 state:resolved]

sunspot/lib/sunspot/query/sort.rb

@@ -90,16 +90,6 @@ def to_param
           "score #{direction_for_solr}"
         end
       end
-
-      # 
-      # A DistanceSort sorts by distance from the origin coordinates of a
-      # geographical distance search.
-      #
-      class DistanceSort < Abstract
-        def to_param
-          "geo_distance #{direction_for_solr}"
-        end
-      end
     end
   end
 end

sunspot/spec/api/query/ordering_pagination_spec.rb

@@ -71,6 +71,14 @@
     end.should raise_error(ArgumentError)
   end
 
+  it 'throws an UnrecognizedFieldError if :distance is given for sort' do
+    lambda do
+      session.search Post do
+        order_by :distance, :asc
+      end
+    end.should raise_error(Sunspot::UnrecognizedFieldError)
+  end
+
   it 'does not allow ordering by multiple-value fields' do
     lambda do
       session.search Post do