Update __init__.py
Update __init__.py

Cleanup:
* Unused import
* Unused assignment.
* Incorrectly named use of var. (read_preference != read_pref)
knowsis/flask-pymongo,hbldh/flask-pymongo,mcreenan/flask-pymongo,dcrosta/flask-pymongo,xebitstudios/flask-pymongo,Dannnno/flask-pymongo,mcreenan/flask-pymongo