$entity_type = 'entity_type_name';
$field_name = 'field_name';
$bundle = 'bundle_name';

// Delete field instance.
if ($instance = field_info_instance($entity_type, $field_name, $bundle))  {
  field_delete_instance($instance, TRUE);
  field_purge_batch(1);
}

// Delete field.
field_delete_field($field_name);
field_purge_batch(1);