Why would a hacker add this code to each post, and how to do mass cleanup?

As I was using MariaDB, the following update did the fix: 

update wp_posts set post_content = REGEXP_REPLACE(post_content,'(*CRLF)<noindex>.*</noindex> ','')

See https://dba.stackexchange.com/questions/234774/mariadb-multiline-regex/234778#234778

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