powerpc/32: Drop a stale comment about reservation of gigantic pages

A comment about the reservation of gigantic pages was left in MMU_init()
after commit 79cc38de ("powerpc/mm/hugetlb: Add support for
reserving gigantic huge pages via kernel command line")

Remove it.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/959d77be630b9b46a7458f0fbd41dc3a94ec811a.1661938317.git.christophe.leroy@csgroup.eu