To remove a field from a particular bundle you can use [`field_delete_instance()`][1]

> Marks a field instance and its data for deletion.

Example:

    function my_module_update_7001() {
      if ($instance = field_info_instance('node', 'field_name', 'page'))  {
        field_delete_instance($instance, TRUE);
        field_purge_batch(1);
      }
    }

To remove a field from the system completely, you can use [`field_delete_field()`][2]

> Marks a field and its instances and data for deletion.

Example:

    function my_module_update_7001() {
      field_delete_field('field_name');
      field_purge_batch(1);
    }

The fields/instances are only marked for deletion, the data will actually be purged during subsequent cron runs. To purged it manually execute:

    field_purge_batch(1);

[1]: http://api.drupal.org/api/drupal/modules%21field%21field.crud.inc/function/field_delete_instance/7
[2]: http://api.drupal.org/api/drupal/modules%21field%21field.crud.inc/function/field_delete_field/7