tomoyo: Use str_yes_no()

Remove the local yesno() implementation and adopt the str_yes_no() from
linux/string_helpers.h.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220126093951.1470898-11-lucas.demarchi@intel.com