iwlwifi: remove incorrect comment in pnvm

We use this driver as a backport that also runs on older kernels (as
part of the backports project).  So we use some checks to backport or
prevent code from compiling in incompatible kernel version.

When I took one of the PNVM patches from the backport, I accidentally
left the comment that a certain part of the code doesn't work in older
kernels.  This obviously should never be valid for the mainline.
Remove this comment.

Reported-by: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/iwlwifi.20210211223049.40d545a0fa89.I04793aaa5312b926335c8db32131f000432df511@changeid