[Data] Remove unused requirements_legacy_compat.txt requirements file

[bveeramani]

Why are these changes needed?

We previously tested that you could install ray[tune,data] with older versions of packages like PyArrow and Torch. We don't run this test anymore, so I'm removing the now-unused requirements file.

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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 :(