The PlainTextConverter module has an unused mammoth import that was copied from DocxConverter. This is dead code and misleading — it suggests PlainTextConverter has a dependency on mammoth when it doesn't. The import also stores _dependency_exc_info on import failure, but never checks it, creating an unused variable that serves no purpose.

Fix

Remove the mammoth import block and the associated _dependency_exc_info variable from _plain_text_converter.py.