Do not overwrite api key

Author: umitanuki

alpaca_trade_api/polygon/rest.py

@@ -17,7 +17,6 @@ class REST(object):
 
     def __init__(self, api_key, staging=False):
         self._api_key = get_polygon_credentials(api_key)
-        self._api_key = api_key
         self._staging = staging
         self._session = requests.Session()