Don't mark auto-generated types as extension members.

Don't mark auto-generated types as extension members.

Relation rowtypes and automatically-generated array types do not need to
have their own extension membership dependency entries.  If we create such
then it becomes more difficult to remove items from an extension, and it's
also harder for an extension upgrade script to make sure it duplicates the
dependencies created by the extension's regular installation script.

I changed the code in such a way that this happened in commit
988cccc620dd8c16d77f88ede167b22056176324, I think because of worries about
the shell-type-replacement case; but that cure was worse than the disease.
It would only matter if one extension created a shell type that was
replaced with an auto-generated type in another extension, which seems
pretty far-fetched.  Better to make this work unsurprisingly in normal
cases.

Report and patch by Robert Haas, comment adjustments by me.