README.txt

Source: https://www.bluespark.com/blog/uninstalling-and-purging-field-modules-all-once

The job is divided in three parts: The data definition, field data purge and module list clean.

In the data definition task we provide all the required data we need to perform the task, the name of the field to delete, and given that 
information, we get the field_info array and the name of the module to be uninstalled. Finally, field_delete_field() is executed.

After that the field data is purged in the batch body, and since we don't know how much data we will have to purge, we remove just 100 
database rows per batch execution. After each purge we check if all the data has been removed to decide if we have to remove more data 
from the database or continue to the final part.

Once all the data and metadata related to the module is removed from the database, the Drupal field types dependency is gone and we are 
granted the ability to disable and uninstall our module cleanly. Finally, we can drop the empty field_deleted_data_XX and 
field_deleted_revision_XX tables to keep our database clean.

Using this approach, we have two key benefits: a. we are sure that the module is disabled and our database is clean, and b. we are 
confident that we can remove the module from our repository, given that in the next deploy we won't get any dependency conflict with that 
module.

my_module.install.php

<?php
/**
 * Removes my_field and uninstalls its module.
 */
function my_module_update_70XX(&$sandbox) {
  // If this is the first pass through this update function then set some variables.
  if (!isset($sandbox['total'])) {
    $sandbox['field_name'] = 'my_field';
    $sandbox['field'] = field_info_field($sandbox['field_name']);
    $sandbox['module'] = array($sandbox['field']['module']);
    field_delete_field($sandbox['field_name']);
  }
  
  field_purge_batch(100);
  $sandbox['#finished'] = field_read_field($sandbox['field_name'], array('include_deleted' => 1)) ? 0 : 1;
  if ($sandbox['#finished'] === 1) {
    module_disable($sandbox['module']);
    drupal_uninstall_modules($sandbox['module']);
    db_drop_table('field_deleted_data_' . $sandbox['field']['id']);
    db_drop_table('field_deleted_revision_' . $sandbox['field']['id']);
  }
}