JS codestyle: no-useless-concat

Style error information

https://eslint.org/docs/rules/no-useless-concat

Remark

Code refactoring.

How to Review

## 1. Apply Patch
## 2. Review Code Changes
## 3. Confirm no Code Standard Errors
yarn & yarn lint:core-js-passing
## 4.1 If `NO` errors, mark the issue as `Reviewed & tested by the community` (Don't be shy, we're all friendly)
## 4.2 If `HAS` errors, fix it and upload a new patch (Just do it and you can!!!)

Background

- #2912962: Step 1 JS codestyle: [meta] Fix JS coding standards in core

- We adapted the airbnb coding standard (#2815077: Adopt airbnb javascript style guide v14.1 as new baseline javascript coding standards for Drupal 8 core and contrib), but we are not fully compliant to it yet.

More Information

- Using ES6 in Core
https://www.drupal.org/node/2815083

- To find JS code standard errors stats
yarn & yarn lint:core-js-stats

Valuable Follow-up

- N/A