[kernel] Missed changes from code review feedback.

Land a few additional changes which fell out of code review feedback,
but somehow failed to get uploaded before landing.  Specifically...

++ Instead of discussing ways to approach a TODO in the TODO comment
   itself, file a bug instead and have the discussion there.
++ Remove the constexpr = default constructor from the custom deleter
   for MessagePackets.  While it is part of the std::default_delete,
   it is currently serving no valuable purpose in our version as far
   as I can tell, so just remove it instead of cargo culting the std::
   library.

Test: Build and runtests on QEMU
Change-Id: I9c4f0f859184f68a4ac0f6c52929a1c72cfa4806