iwlwifi: pcie: remove outdated comment about PCI RTPM reference

This comment was only related to the PCI RTPM implementation, which
has been removed a while ago, and is not meaningless.  Remove it.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20200424194456.f362e4560a3b.I8d858c6c9a6c98b45d2195dfe28dabe0286c8a83@changeid