BURs: remove dead error handling code.

This is a 'this should never happen' error. This error is only raised if
we add a new bulk update request type and forget to handle it somewhere.

test/unit/bulk_update_request_test.rb

@@ -458,10 +458,6 @@ class BulkUpdateRequestTest < ActiveSupport::TestCase
 
       should "gracefully handle validation errors during approval" do
         @req.stubs(:update!).raises(BulkUpdateRequestProcessor::Error.new("blah"))
-        assert_difference("ForumPost.count", 1) do
-          @req.approve!(@admin)
-        end
-
         assert_equal("pending", @req.reload.status)
       end