Open the AZW3 with the calibre editor, unmark the cover image, and delete the file, then save.
Rinse. Repeat.
MOBI files cannot be edited, nohow noway. You will need to convert it to a more modern format, or use KindleUnpack (which is more work than converting to EPUB/AZW3).
I know of no way to apply that in bulk as you can for EPUB.
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