Implements hook_field_storage_purge_field().
All field data items and instances have already been purged, so all that is left is to delete the table.
function field_sql_storage_field_storage_purge_field($field) {
$table_name = _field_sql_storage_tablename($field);
$revision_name = _field_sql_storage_revision_tablename($field);
db_drop_table($table_name);
db_drop_table($revision_name);
}