I have upgraded a website to drupal 8 that I found a field created and I need to delete that field as it became useless.

The field is locked, and this prevents me delete it from the interface. I tried to delete it using field_purge_field(), but it didn't work either.

The answer for How to properly delete fields, programmatically? didn't work for me.

How do I delete fields that result locked? How do I unlock them before deleting them?