Revert "checkpatch: add check for keyword 'boolean' in Kconfig definitions"

This reverts commit 327953e9.

You cannot use 'boolean' since commit b92d804a ("kconfig: drop
'boolean' keyword").

This check is no longer needed.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Joe Perches <joe@perches.com>