Remove kwarg assignments that are later deleted.

It would be nice to know what these assignments are meant to do since they are being used on recent ReQL terms.
I get an invalid syntax error when running the tests on this line (only in Python 2, not Python 3):

```
  File "/home/ssd2/grey/review/rethinkdb/build/drivers/python/rethinkdb/ast.py", line 83
    self.optargs = {k: expr(v) for k, v in dict_items(optargs)}
                                 ^
SyntaxError: invalid syntax
```