KVM: PPC: Book3S HV: remove ISA v3.0 and v3.1 support from P7/8 path

POWER9 and later processors always go via the P9 guest entry path now.
Remove the remaining support from the P7/8 path.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210528090752.3542186-33-npiggin@gmail.com