Remove ArgumentError.

It is untested. There is no `@frame` variable. Presumably it is supposed
to be `@framing`, but changing that shows that some of the tests *are*
setting frame twice.

I don't see why this level of strictness is necessary. If someone
disagrees, they should add a test for this behaviour and make the other
tests pass.

lib/arel/nodes/window.rb

@@ -17,7 +17,6 @@ def order *expr
       end
 
       def frame(expr)
-        raise ArgumentError, "Window frame cannot be set more than once" if @frame
         @framing = expr
       end
 
@@ -75,4 +74,4 @@ def initialize(expr = nil)
       end
     end
   end
-end
+end