[Popup]: Clean up body style class on disconnect

[MarcusNotheis]

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When removing an open Popup with the modal property set or when removing an open Dialog from the DOM, the style class sapUiBLyBack is not removed from the body tag.
This is causing inconsistencies in our application.

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Implement the disconnectedCallback of the Web Components lifecycle in order to clean up the style.

Describe alternatives you've considered
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None.

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Happy to submit a Pull Request if desired 😄

[fifoosid]

The issue was solved.

PS: Would like to hear feedback if the solution fits your use case.