[Component Rule] Revert es6 variable declarations

[Den4ik]

Description (*)

Currently, we support IE 11 browser that not good supports ES6 features.

We can't add es6 to eslint cause it's will mean that we fully support ES6 but currently, it isn't.
Let and const declaration are already supported by IE11, but again, we should use all features of ES6 or don't use anything.

This PR revert es6 variable declaration via let/const

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @Den4ik. Thank you for your contribution
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[magento-engcom-team]

Hi @dmytro-ch, thank you for the review.
ENGCOM-4597 has been created to process this Pull Request

[orlangur]

Is IE11 support going to be dropped any soon?

[VasylShvorak]

✔️ QA passed

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