Fix `--bs-form-check-bg` definition

Related issues

Closes #37620 (closed)

Description

Customizing $input-bg change $*-input-bg values. The problematic one was $form-check-input-bg that tried to apply escape-svg() on it. This PR suggests to simply remove unnecessary call to escape-svg() since we're handling colors and not images in this case.

Type of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• All new and existing tests passed

Testing

• All use cases depending on $form-check-input-bg still work correctly
• Modifying $input-bg to #fd0 (for example) doesn't cause anymore any compilation issues