Why I can't I drop field_data and field_revision table in phpmyadmin?
@hampercm explained why directly deleting records and tables from database is not recommended. It is so true that everyone is encouraged to use admin UI or Drupal API to remove the fields and field instances.

But, I guess you know what the best practise is and wanna do that by SQL query with some reason (e.g. company policy. YES, I faced this stupid requirement before)

You are on the right track. Drupal save field and field instances config in field_config and field_config_instance. BUT, Drupal also cache these data in cache table.

So, you need to truncate the cache table.