query: no prepared param for get_db_prep_lookup

Looking up in Django codebase, the `prepared` parameter for
get_db_prep_lookup appears nowhere.  Its existence is harmless since
it's an optional argument, but confusing nonetheless.

bitfield/query.py

@@ -15,7 +15,7 @@ def process_lhs(self, qn, connection, lhs=None):
         params.extend(self.get_db_prep_lookup(self.rhs, connection)[1])
         return lhs_sql, params
 
-    def get_db_prep_lookup(self, value, connection, prepared=False):
+    def get_db_prep_lookup(self, value, connection):
         v = value.mask if isinstance(value, (BitHandler, Bit)) else value
         return super(BitQueryLookupWrapper, self).get_db_prep_lookup(v, connection)