Issue #1019374 by Jody Lynn: Delete book variables on hook_uninstall().

modules/book/book.install

@@ -17,6 +17,10 @@ function book_install() {
  * Implements hook_uninstall().
  */
 function book_uninstall() {
+  variable_del('book_allowed_types');
+  variable_del('book_child_type');
+  variable_del('book_block_mode');
+
   // Delete menu links.
   db_delete('menu_links')
     ->condition('module', 'book')