[SkyServe] Not add default value of {up,down}scale_delay_seconds in service spec

[cblmemo]

Currently, the SkyServiceSpec will add the fields {up,down}scale_delay_seconds even if there are no autoscaling enabled. This PR fixed the issue.

To reproduce:

>>> import sky
>>> sky.serve.SkyServiceSpec.from_yaml_config({'readiness_probe': '/', 'replicas': 1}).to_yaml_config()
{'readiness_probe': {'path': '/', 'initial_delay_seconds': 1200}, 'replica_policy': {'min_replicas': 1, 'upscale_delay_seconds': 300, 'downscale_delay_seconds': 1200}}

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
  • The reproducible script shown above; now returning

>>> import sky
>>> sky.serve.SkyServiceSpec.from_yaml_config({'readiness_probe': '/', 'replicas': 1}).to_yaml_config()
{'readiness_probe': {'path': '/', 'initial_delay_seconds': 1200}, 'replica_policy': {'min_replicas': 1}}

• All smoke tests: pytest tests/test_smoke.py
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• Backward compatibility tests: bash tests/backward_comaptibility_tests.sh

[MaoZiming]

LGTM.
alternatively, we can check whether autoscaler is enabled in service_spec.