powerpc/mm: Remove stale comment about the DART hole

The code to fix the problem it describes was removed in commit
c40785ad

 ("powerpc/dart: Use a cachable DART"), and it uses the
stupid comment style. Away it goooooooooooooes!

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>