x86: Drop KERNEL_IMAGE_START

We have KERNEL_IMAGE_START and __START_KERNEL_map which both contain the
start of the kernel text mapping's virtual address. Remove the prior one
which has been replicated a lot less times around the tree.

No functionality change.

Signed-off-by: Borislav Petkov <bp@alien8.de>
Link: http://lkml.kernel.org/r/1362428180-8865-3-git-send-email-bp@alien8.de
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>