kstrtox: remove strtobool()

The conversion from strtobool() to kstrtobool() is completed.  So
strtobool() can now be removed.

Link: https://lkml.kernel.org/r/87e3cc2547df174cd5af1fadbf866be4ef9e8e45.1694878151.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>