Simplify event trigger support checking functions

We have in event_trigger.c two functions
EventTriggerSupportsObjectType() and EventTriggerSupportsObjectClass()
that return whether a given object type/class supports event triggers.
Maybe there was a real differentiation there once, but right now it
seems to me that *all* object types/classes support event triggers,
except: (1) shared objects and (2) event triggers themselves. I think
we can write that logic more explicitly and compactly and don't have to
give the false illusion that there is a real choice to be made by the
implementer here.

The only drawback in terms of code robustness is that someone adding a
new shared object type would have to remember to add it to
EventTriggerSupportsObjectType(). Maybe we could add a "object type is
shared" function somehow, similar to IsSharedRelation(), to make that
easier. OTOH, someone doing that would probably go around and grep for,
say, OBJECT_TABLESPACE and find relevant places to update that way.

Thoughts?