Re: some more include removal from headers

I pushed this now, after also removing the #include instrument.h from
execnodes.h.

One I purposefully didn't remove from execnodes is reltrigger.h, though
it's quite an easy change.  I think what we should do here is rethink
the relationship between rel.h, trigger.h and reltrigger.h.  It looks
weird to me that structs like TriggerData (used for firing triggers) are
in a header file under src/include/commands.  Maybe revert b93f5a5673b4
entirely.

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/
"Pido que me den el Nobel por razones humanitarias" (Nicanor Parra)