powerpc/64s: Remove spurious fault flushing for NMMU

Commit 6d8278c4

 ("powerpc/64s/radix: do not flush TLB on spurious
fault") removed the TLB flush for spurious faults, except when a
coprocessor (nest MMU) maps the address space. This is not needed
because the NMMU workaround in the PTE permission upgrade paths
prevents PTEs existing with less restrictive access permissions than
their corresponding TLB entries have.

Remove it and replace with a comment.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220525022358.780745-4-npiggin@gmail.com