powerpc/irq: Remove stale and misleading comment

The comment claims we will call host->ops->map() to update the flags if
we find a previously established mapping, but we never did.  We used
to call remap, but that call was removed in da05198002 (powerpc: Remove
irq_host_ops->remap hook).

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>