target-ppc: Remove unused mmu models from ppc_tlb_invalidate_one

ppc_tlb_invalidate_one() has a big switch handling many different MMU
types.  However, most of those branches can never be reached:

It is called from 3 places: from remove_hpte() and h_protect() in
spapr_hcall.c (which always has a 64-bit hash MMU type), and from
helper_tlbie() in mmu_helper.c.

Calls to helper_tlbie() are generated from gen_tlbiel, gen_tlbiel and
gen_tlbiva.  The first two are only used with the PPC_MEM_TLBIE flag,
set only with 32-bit or 64-bit hash MMU models, and gen_tlbiva() is
used only on 440 and 460 models with the BookE mmu model.

These means the exhaustive list of MMU types which may call
ppc_tlb_invalidate_one() is: POWERPC_MMU_SOFT_6xx, POWERPC_MMU_601,
POWERPC_MMU_32B, POWERPC_MMU_SOFT_74xx, POWERPC_MMU_64B, POWERPC_MMU_2_03,
POWERPC_MMU_2_06, POWERPC_MMU_2_07 and POWERPC_MMU_BOOKE.

Clean up by removing logic for all other MMU types from
ppc_tlb_invalidate_one().

This means that ppc4xx_tlb_invalidate_virt() now has no callers, or rather,
makes it obvious that it has no callers.  So, we remove that function as
well.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
