[Backport] fix: remove old code in tabs, always set tabindex to 0 when tabs are …

[DanielRuf]

Original Pull Request

#19326
…initialized

Description (*)

There seems to be old code which is duplicate. We always want to set the tabindex to 0 so it can be removed.

Fixed Issues (if relevant)

1. magento/magento2#<issue_number>: Issue title
2. ...

Manual testing scenarios (*)

1. ...
2. ...

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 @DanielRuf. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento-engcom-team give me test instance - deploy test instance based on PR changes
• @magento-engcom-team give me 2.2-develop instance - deploy vanilla Magento instance

For more details, please, review the Magento Contributor Assistant documentation

[magento-engcom-team]

Hi @VladimirZaets, thank you for the review.
ENGCOM-3548 has been created to process this Pull Request

[magento-engcom-team]

Hi @DanielRuf. Thank you for your contribution.
We will aim to release these changes as part of 2.2.8.
Please check the release notes for final confirmation.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.