server: remove deprecated UpgradeOldPrivValFile

[melekes]

Description

OldFilePV was deprecated in Tendermint Core v0.28 and will be removed in
upcoming v0.34.5 release.

---

For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

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For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #6079 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6079   +/-   ##
=======================================
  Coverage   54.43%   54.43%           
=======================================
  Files         431      431           
  Lines       26157    26156    -1     
=======================================
  Hits        14239    14239           
+ Misses      10931    10930    -1     
  Partials      987      987           

[alessio]

I believe this is worth the addition of a changelog item. Looks good otherwise 👍

[alexanderbez]

ACK 🎉