Check if we need $clipboardHolder

[scofalik]

📝 Provide a description of the improvement

This is a follow-up for #9460.

To enable marker conversion for model document fragments, $documentFragment schema item was introduced. The properties of that item are same as for $clipboardHolder. Both of these artificial parents have same purpose - allow for converting everything that's there as is. Let's check if we still need $clipboardHolder after introducing $documentFragment.

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