The `term_exists()` check is not needed because of existing dupe
checks in `wp_insert_term()`. Furthermore, `term_exists()` conflates
term names and sanitized slugs, so incorrectly marks terms like
'C' and 'C+' as duplicates of one another.
Props garyc40, SergeyBiryukov, kovshenin, MikeHansenMe.
Fixes#16567.
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