perf: fix memory leak due to instance reference in validators

[sushantdhiman]

Pull Request check-list

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• Does npm run test or npm run test-DIALECT pass with this change (including linting)?
• Does the description below contain a link to an existing issue (Closes #[issue]) or a description of the issue you are solving?
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Description of change

Fix a memory leak due to uncollected reference to modelInstance in isImmutable validator

Before

After

[codecov]

Codecov Report

Merging #9973 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #9973      +/-   ##
==========================================
- Coverage   96.39%   96.38%   -0.01%     
==========================================
  Files          63       63              
  Lines        9395     9389       -6     
==========================================
- Hits         9056     9050       -6     
  Misses        339      339

Impacted Files	Coverage Δ
lib/instance-validator.js	97.54% <100%> (-0.04%)	⬇️
lib/utils/validator-extras.js	97.56% <100%> (-0.22%)	⬇️

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