Clean up unused code in BibEntry class

[Jennnyyy]

While reviewing BibEntry, I noticed that two overloads of withSharedBibEntryData existed:

public BibEntry withSharedBibEntryData(int sharedId, int version) {
    sharedBibEntryData.setSharedID(sharedId);
    sharedBibEntryData.setVersion(version);
    return this;
}

public BibEntry withSharedBibEntryData(SharedBibEntryData sharedBibEntryData) {
    sharedBibEntryData = sharedBibEntryData;
    return this;
}

The second method contained a self-assignment bug (sharedBibEntryData = sharedBibEntryData;) and therefore did not update the field. After searching the codebase, I found that this method is not used anywhere.

What this PR does
Removes the unused and incorrect withSharedBibEntryData(SharedBibEntryData) method.
The reason to remove: Unused code increases complexity and makes future refactoring harder. Removing this also improves maintainability.

Steps to test

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• [/] I manually tested my changes in running JabRef (always required)
• [/] I added JUnit tests for changes (if applicable)
• [/] I added screenshots in the PR description (if change is visible to the user)
• [/] I described the change in CHANGELOG.md in a way that is understandable for the average user (if change is visible to the user)
• [/] I checked the user documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request updating file(s) in https://github.com/JabRef/user-documentation/tree/main/en.

[koppor]

Maybe used at #11879, but I can re-add if needed.