chore: cleanup

[SimonSchick]

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• Does npm run test or npm run test-DIALECT pass with this change (including linting)?
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Description of change

Fixes #10275 I think

This is another small scale cleanup PR.

• Replace some loops with Object.assign
• Replace all usage of spread with native array spread
• Use object literals in various places
• Avoid aliasing of this

[codecov]

Codecov Report

Merging #10276 into master will decrease coverage by <.01%.
The diff coverage is 98.07%.

@@            Coverage Diff             @@
##           master   #10276      +/-   ##
==========================================
- Coverage   96.28%   96.27%   -0.01%     
==========================================
  Files          68       68              
  Lines        9797     9772      -25     
==========================================
- Hits         9433     9408      -25     
  Misses        364      364

Impacted Files	Coverage Δ
lib/sequelize.js	96.36% <ø> (ø)	⬆️
lib/model.js	96.77% <100%> (-0.01%)	⬇️
lib/dialects/mysql/connection-manager.js	96.36% <100%> (-0.19%)	⬇️
lib/associations/has-one.js	97.24% <100%> (ø)	⬆️
lib/dialects/mssql/connection-manager.js	88.46% <100%> (-0.15%)	⬇️
lib/dialects/abstract/query-generator.js	97.63% <100%> (-0.01%)	⬇️
lib/associations/has-many.js	97.64% <100%> (-0.07%)	⬇️
lib/associations/belongs-to-many.js	97.61% <96.96%> (-0.11%)	⬇️

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[sushantdhiman]

odd indentation

[sushantdhiman]

Thanks @SimonSchick 👍