Taxonomy: Eliminate redundant and inaccurate dupe check when creating categories from post.php.

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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git-svn-id: https://core.svn.wordpress.org/trunk@39577 1a063a9b-81f0-0310-95a4-ce76da25c4cd