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

src/wp-admin/includes/schema.php

@@ -51,7 +51,7 @@ function wp_get_db_schema( $scope = 'all', $blog_id = null ) {
  slug varchar(200) NOT NULL default '',
  term_group bigint(10) NOT NULL default 0,
  PRIMARY KEY  (term_id),
- UNIQUE KEY slug (slug),
+ KEY slug (slug),
  KEY name (name)
 ) $charset_collate;
 CREATE TABLE $wpdb->term_taxonomy (