Fix #8384 - Add layout_def_key to return_url querystring in sub_p_rem

[marin-h]

Add layout_def_key with the value provided by get_layout_def_key() to query string in return_url variable generated in function sub_p_rem used by the SubPanelRemoveButton widget.

Description

Fixes #8384

Motivation and Context

See #8384

How To Test This

1. In a custom module or an extension for a core module, add a non-default layout_def_key to layoutdefs or subpaneldefs with a subpanel definition on it that uses the widget SubPanelRemoveButton.
2. Access the view that renders this new subpanel.
3. Add a record to the subpanel.
4. Delete the record using SubPanelRemoveButton.
5. Record and relationship are deleted.
6. Subpanel loads succesfully.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Final checklist

• My code follows the code style of this project found here.
• My change requires a change to the documentation.
• I have read the How to Contribute guidelines.

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