powerpc/64s/exception: machine check fwnmi remove HV case

fwnmi does not trigger in HV mode, so remove always-true feature test.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190802105709.27696-2-npiggin@gmail.com