MDL-74285 cachestore_redis: Unable to delete a Redis cache instance

When deleting a cache instance, it tried to purge it, even though
purge only makes sense for an individual definition (not the whole
instance). As a result, it errored.

cache/stores/redis/lib.php

@@ -468,7 +468,6 @@ class cachestore_redis extends cache_store implements cache_is_key_aware, cache_
      * Cleans up after an instance of the store.
      */
     public function instance_deleted() {
-        $this->purge();
         $this->redis->close();
         unset($this->redis);
     }