clean up : remove re-export of overlay (moved since 47.0.0)

[Rich-T-kid]

Which issue does this PR close?

• n/a

Rationale for this change

see https://github.com/apache/datafusion/pull/24544/changes#r3826460540

What changes are included in this PR?

removes overlay file and import in datafusion/functions/src/mod.rs

Are these changes tested?

n/a

Are there any user-facing changes?

yes, but the import was deprecated for over 6 releases ago.

[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-functions v55.0.0 (current)
       Built [  38.224s] (current)
     Parsing datafusion-functions v55.0.0 (current)
      Parsed [   0.089s] (current)
    Building datafusion-functions v55.0.0 (baseline)
       Built [  30.925s] (baseline)
     Parsing datafusion-functions v55.0.0 (baseline)
      Parsed [   0.089s] (baseline)
    Checking datafusion-functions v55.0.0 -> v55.0.0 (no change; assume patch)
     Checked [   0.547s] 223 checks: 221 pass, 2 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_functions::string::overlay, previously in file /home/runner/work/datafusion/datafusion/target/semver-checks/git-apache_main/7f66ea7cb840e6752ccdedef94ed84cf1cacaa5b/datafusion/functions/src/string/overlay.rs:18

--- failure struct_missing: pub struct removed or renamed ---

Description:
A publicly-visible struct cannot be imported by its prior path. A `pub use` may have been removed, or the struct itself may have been renamed or removed entirely.
        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/struct_missing.ron

Failed in:
  struct datafusion_functions::string::overlay::OverlayFunc, previously in file /home/runner/work/datafusion/datafusion/target/semver-checks/git-apache_main/7f66ea7cb840e6752ccdedef94ed84cf1cacaa5b/datafusion/functions/src/core/overlay.rs:61

     Summary semver requires new major version: 2 major and 0 minor checks failed
    Finished [  71.331s] datafusion-functions

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.43%. Comparing base (26b40dd) to head (25c43dd).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24635      +/-   ##
==========================================
- Coverage   81.43%   81.43%   -0.01%     
==========================================
  Files        1118     1118              
  Lines      399414   399414              
  Branches   399414   399414              
==========================================
- Hits       325278   325263      -15     
- Misses      55145    55154       +9     
- Partials    18991    18997       +6     

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[alamb]

Thank you @Rich-T-kid