target-mips: remove misleading comments in translate_init.c

PABITS are not hardcoded to 36 bits and we do not model 59 PABITS (which is
the architectural limit) in QEMU.

Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
