schema: remove unused function

[pjonsson]

---

📚 Documentation preview 📚: https://datacube-explorer--841.org.readthedocs.build/en/841/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.43%. Comparing base (5737151) to head (0931865).
⚠️ Report is 18 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #841      +/-   ##
===========================================
+ Coverage    84.36%   84.43%   +0.06%     
===========================================
  Files           35       35              
  Lines         4253     4246       -7     
  Branches       537      536       -1     
===========================================
- Hits          3588     3585       -3     
+ Misses         469      465       -4     
  Partials       196      196              

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[omad]

The date related function was last referenced in August 2018 😮

The PG Indexes one was only June last year.