PYCBC-140: fix dfl_fmt refcount issues/cleanup ctor

- Remove redundant code in constructor
- Fix bug where dfl_fmt was decref'd twice

Change-Id: Ib76fe5d1a4d7854e565097f9f06c69378d139edf
Reviewed-on: http://review.couchbase.org/27063
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>