fix: remove disabled attribute on region list

[DanielRuf]

Description

By default when a list is not populated at the beginning (eg for UK) and when we switch to another country (eg Germany) and get a populated list, the list is still disabled and we can not select any option.

Fixed Issues (if relevant)

2. ...

Manual testing scenarios

1. go to /customer/account/create/
2. switch from a country with an empty region list to a country with a region list
3. try to select a region (does not work as the input is still disabled)

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:

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[DanielRuf]

@magento-engcom-team give me 2.2.5 instance

[magento-engcom-team]

Hi @DanielRuf. Thank you for your request. I'm working on Magento 2.2.5 instance for you

[magento-engcom-team]

Hi @DanielRuf, here is your Magento instance.
Admin access: https://i-16955-2-2-5.engcom.dev.magento.com/admin
Login: admin Password: 123123q
Instance will be terminated in up to 3 hours.

[magento-engcom-team]

Hi @DanielRuf. Thank you for your contribution.
We will aim to release these changes as part of 2.2.7.
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.