powerpc/64s: Remove lost/old comment

The bulk of this was moved/reworded in:
  57f26649 ("powerpc: Use gas sections for arranging exception vectors")

And now appears around line 700 in arch/powerpc/kernel/exceptions-64s.S.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220928130941.1732818-1-mpe@ellerman.id.au