Remove tag duplicates

Prevention of getting this error:

```
ActiveRecord::RecordNotUnique: PG::Error: ERROR:  could not create unique index "index_tags_on_name"
DETAIL:  Key (name)=(XSS') is duplicated.
: CREATE UNIQUE INDEX  "index_tags_on_name" ON "tags"  ("name")
```

This migration fails if there are duplicates:
https://github.com/mbleigh/acts-as-taggable-on/blob/v3.5.0/db/migrate/2_add_missing_unique_indices.rb#L3

See merge request !623