Remove trigram indexes for "ci_runners"

These indexes are only used when you search for runners in the admin
interface. This operation is so rarely used that it does not make sense
to slow down every update in order to update the GIN trigram indexes.

Removing these indexes should speed up queries such as those used for
updating the last contact time of CI runners. Locally the timings of
this query were reduced from ~50 ms to ~25 ms:

    UPDATE ci_runners SET updated_at = now(), contacted_at = now();