Drop `DSTACK_FF_AUTOCREATED_FLEETS_ENABLED` feature flag

DSTACK_FF_AUTOCREATED_FLEETS_ENABLED is a feature flag that enables legacy autocreated fleets: If there are no fleet suitable for the run, a new fleet is created automatically instead of an error.

Dropping this legacy behavior will simplify implementing #3665 as we'll no longer need to support  the create-fleet-on-run-submission path.