[Data] Remove read task warning if size bytes not set in metadata

[bveeramani]

Why are these changes needed?

Datasource.get_read_tasks() returns a list of ReadTask, where each ReadTask encapsulates a data reading function and it's associated output metadata. If a serialized ReadTask is more than 100KB and size_bytes isn't set in the output metadata (which is often the case), then Ray Data emits a warning per read task:

WARNING: the read task size (288451 bytes) is larger than the reported output size of the task (None bytes). This may be a size reporting bug in the datasource being read from.

This warning isn't helpful and usually doesn't indicate an actual issue, so I'm removing it.

Related issue number

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• 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 added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• 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 :(