Don't force enable "Use system value" checkboxes

[likemusic]

Description

"Use system value"

Fixed Issues (if relevant)

No

Manual testing scenarios

1. Create plugin for \Magento\Config\Block\System\Config\Form\Field with beforeRender() method with code:

$element->setDisabled(true);
$element->setIsDisableInheritance(true);

2. Open System config page with field disabled by plugin.
   Expected: Field is disabled.
   Current: Field is enabled.

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 @likemusic. 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.