kernel.bbclass: remove references to update-modules

Since update-modules is now obsolete, remove it from the bbclass.

[YOCTO #3598]

(From OE-Core rev: 323146c7fdf1c47241f3fd5adab2cb3c6d24b56c)

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Sergey Ignatov <sergey_ignatov@mentor.com>