Delete all instances and the field in Drupal 7

delete-field.php

$field_name = '';
$entity_name = '';

$entity = entity_get_info($entity_name);

foreach ($entity['bundles'] as $key => $value) {
  $field_instance = field_info_instances($entity_name, $key);
  $instance = $field_instance[$field_name];
  field_delete_instance($instance);
}

try {
  field_delete_field($field_name);
}
catch (Exception $e) {
  dpm($e->getMessage());
  dpm('Field is probably present on other entities');
}