build-appliance-image: Drop kernel module handling

kernel-devsrc is a dependency of the image so we no longer need the symlink
creation code or the module-base inherit/dependency as it is included in
that recipe. The KERNEL_VERSION usage was broken anyway as the module
usage would have needed a:

do_image[depends] += "build-appliance-image:do_configure"

which wasn't present so it was indeterminate if KERNEL_VERSION was set
correctly.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>