[air/tune] Remove postprocess_checkpoint

[krfricke]

Why are these changes needed?

The postprocess checkpoint method was introduced to be able to add data to function runner checkpoint directories before they are uploaded to external (cloud) storage. Instead, we should just use the existing separation of save_checkpoint() and save().

Related issue number

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[Yard1]

Looks good, thanks!

[xwjiang2010]

Thanks Kai!

[amogkam]

Thanks @krfricke!