powerpc/64/kexec: Remove BookE special default_machine_kexec_prepare()

The only difference is now the TCE table check which doesn't need
to be ifdef'ed out, it will basically do nothing on BookE (it is
only useful for ancient IBM machines).

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>