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

> Marks a field instance and its data for deletion.

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.

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

[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