Refactor: controller: Improve name of controld_stop_election_timer()

To disambiguate with the controller-internal election_timer.
controld_stop_election_timer() stops the timer that counts down for an
ongoing election. election_timer (formerly election_trigger) counts down
how long to wait for the DC before triggering an election.

Ref T239

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>