Delete wrong comment in mnesia adapter
Delete wrong comment in mnesia adapter

Comment taken from pgsql adapter, obsolete in mnesia adapter since day
1, and already wrong in pgsql adapter.

First mnesia adapter version had separate approach for new id - not
0. See
https://github.com/inaka/sumo_db/pull/129/commits/c7730aa942ea2dbea309d8a294cb0935bd75ebab#diff-7f88a0f03704c76580812ce8392eaa93R62

inaka/sumo_db