#384 Post-Python 3: Remove Title submissions may contain deleted Title IDs

[Ahasuerus]

Remove Title submissions may contain deleted Title IDs if one or more about-to-be deleted titles were deleted by a different editor between the time a publication's list of titles was loaded into "Removal Editor" and the time "Submit Data" was clicked. This isn't a significant issue because the submission review page checks the existence of all to-be-deleted titles and forces a "Hard Reject" error, but it would be nice to check each title's existence in edit/submitrm.py as well.