This is a workaround, not a perfect solution.
Golden rule: work on a copy of your document
Save your document as .fodt (Flat XML ODF Text Document)
Open the resulting .fodt with a text editor
A name to be entered in the index is marked up as follows:
<text:alphabetical-index-mark-start text:id="IMark…" />name<text:alphabetical-index-mark-end text:id="IMark…" />
where the … stands for a unique numeric id,
or, in case you defined specifically the key
<text:alphabetical-index-mark text:string-value="key" />name
Search for the adequate variants, preferably with a regular expression, to delete the XML elements, keeping the “name” ot replacing the parts you need.
When done, reopen the modified .fodt with Writer and save anew as .odt.
Your edit tells the index marks are not in the master document (where you only insert the Alphabetical Index), but in the individual .odt subdocument files.
Apply the above workaround to every subdocument files.
If you have any index marks in the own text of the master (in addition to the Alphabetical Index insertion), don’t apply the workaround because you’ll lose the master property (a master cannot be saved as a fodt without reverting to ordinary document). Instead, remove the index references manually.
To show the community your question has been answered, click the ✓ next to the correct answer, and “upvote” by clicking on the ^ arrow of any helpful answers. These are the mechanisms for communicating the quality of the Q&A on this site. Thanks!
In case you need clarification, edit your question (not an answer which is reserved for solutions) or comment the relevant answer.