Fix Issue #164: generate choices schema without collectionFormat

Author: Леопольд Пекальн

flask_restx/reqparse.py

@@ -307,7 +307,6 @@ def __schema__(self):
             param["collectionFormat"] = "csv"
         if self.choices:
             param["enum"] = self.choices
-            param["collectionFormat"] = "multi"
         return param