minor: Remove empty public module `datafusion::physical_plan::joins::chain

[2010YOUY01]

Which issue does this PR close?

• Closes #.

Rationale for this change

datafusion::physical_plan::joins::chain is a public module with no public member.

See https://docs.rs/datafusion/latest/datafusion/physical_plan/joins/chain/index.html

I think it was made public by mistake, this PR revert this module to private.

What changes are included in this PR?

What is the testing strategy for this PR?

Are there any user-facing changes?

[github-actions]

Thank you for opening this pull request!

Reviewer note: cargo-semver-checks reported the current version number is not SemVer-compatible with the changes in this pull request (compared against the base branch).

Details

     Cloning apache/main
    Building datafusion-physical-plan v55.1.0 (current)
       Built [  40.506s] (current)
     Parsing datafusion-physical-plan v55.1.0 (current)
      Parsed [   0.155s] (current)
    Building datafusion-physical-plan v55.1.0 (baseline)
       Built [  39.092s] (baseline)
     Parsing datafusion-physical-plan v55.1.0 (baseline)
      Parsed [   0.168s] (baseline)
    Checking datafusion-physical-plan v55.1.0 -> v55.1.0 (no change; assume patch)
     Checked [   0.697s] 223 checks: 222 pass, 1 fail, 0 warn, 31 skip

--- failure module_missing: pub module removed or renamed ---

Description:
A publicly-visible module cannot be imported by its prior path. A `pub use` may have been removed, or the module may have been renamed, removed, or made non-public.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.50.0/src/lints/module_missing.ron

Failed in:
  mod datafusion_physical_plan::joins::chain, previously in file /home/runner/work/datafusion/datafusion/target/semver-checks/git-apache_main/19eca4ceba9795040fbfc82d0ebf432501d85c68/datafusion/physical-plan/src/joins/chain.rs:18

     Summary semver requires new major version: 1 major and 0 minor checks failed
    Finished [  82.085s] datafusion-physical-plan

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.90%. Comparing base (38d58ed) to head (db608ac).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25286      +/-   ##
==========================================
- Coverage   81.90%   81.90%   -0.01%     
==========================================
  Files        1134     1134              
  Lines      425217   425217              
  Branches   425217   425217              
==========================================
- Hits       348289   348276      -13     
- Misses      56288    56297       +9     
- Partials    20640    20644       +4     

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