powerpc/powernv/npu: Remove obsolete comment about TCE_KILL_INVAL_ALL

TCE_KILL_INVAL_ALL has moved long ago but the comment was forgotted so
finish the move and remove the comment.

Fixes: 0bbcdb43 "powerpc/powernv/npu: TCE Kill helpers cleanup"
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>