iwlwifi: remove the code under IWLWIFI_PCIE_RTPM

This flag should never be set unless integration work with the
platform is done.  We don't support any platforms officially and don't
plan to do so in the near future, so we can remove this option
entirely in order to avoid having it enabled by mistake.

This has been marked with "depends on EXPERT", so there shouldn't be
many systems running with it set.  And, if there are systems, they
should not be using this flag.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>