Opportunity for code cleanup: Remove deprecated methods registerChild

**Describe the issue**
The Frank!Doc is quite complicated. One of the reasons has to do with method registerChild. This method has multiple overloads, causing multiple ElementRole objects sharing the same element role. This situation requires complicated logic in the Frank!Doc. Method registerChild is deprecated however. If we remove the registerChild methods, we may be able to remove much logic from the Frank!Doc. Doing so will make future development more easy.

**Reporter**
Martijn Dirkse

**To Reproduce**
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2. Click on '....'
3. Scroll down to '....'
4. See error

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**Configuration**
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**Input**
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**Environment**
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**Additional Environment**
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**Additional Context**
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