kconfig: drop 'boolean' keyword

No more users of this keyword.  Drop it according to the notice by
commit 6341e62b

 ("kconfig: use bool instead of boolean for type
definition attributes").

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Luis R. Rodriguez <mcgrof@kernel.org>