Remove UNIQUE key from 'slug' column of terms table.

Each slug is a unique and beautiful snowflake, but let's enforce that
uniqueness elsewhere.

Props hotchkissconsulting.
Fixes #22023.

git-svn-id: https://develop.svn.wordpress.org/trunk@30056 602fd350-edb4-49c9-b593-d223f7449a82