Delete book variables on hook_uninstall

I applied the patch then installed and uninstalled the book module to verify the patch works to remove the book_allowed_types, book_child_type, and book_block_mode variables from the variable table when the module is uninstalled. I found no other book module variables that need to be removed in hook_uninstall().

Patch applies in both D8 and D7, and should be committed to both branches.

Let the bot test it.

Nice clean-up, thanks!

Committed to 8.x and 7.x.