sql/execversion: remove no longer needed V25_2

[yuzefovich]

Note that we keep on using V25_4 since we haven't encountered a case where the exec version bump is required.

Epic: None
Release note: None

[cockroach-teamcity]

This change is 

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

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[github-actions]

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