kernel-fitimage.bbclass: remove it

kernel-fitimage.bbclass: remove it

The integration of the FIT image-related build steps into the kernel
recipe has proven to be not very good. The new implementation with
kernel-fit-image.bbclass fixes some design issues:

* sstate does not work well when a fitImage contains an initramfs. The
  kernel is rebuilt from scratch if the build runs from an empty TMPDIR.
* A fitImage kernel is not available as a package, but all other kernel
  image types are.
* The task dependencies in the kernel are very complex and difficult to
  debug if something goes wrong. As a separate, downstream recipe, this
  is now much easier.

The long storry about this issue is here:
[YOCTO #12912]

Signed-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>