Revert "powerpc/64: Disable use of radix under a hypervisor"

This reverts commit 3f91a89d.

Now that we do have the machinery for using the radix MMU under a
hypervisor, the extra check and comment introduced in 3f91a89d are
no longer correct.  The result is that when booted under a hypervisor
that only allows use of radix, we clear the MMU_FTR_TYPE_RADIX and
then set it again, and print a warning about ignoring the
disable_radix command line option, even though the command line does
not include "disable_radix".

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>