small pg_dump code cleanup

While reviewing Daniel's pg_dump patch [0], I was initially confused
because the return value of getTypes() isn't saved anywhere. Once I found
commit 92316a4, I realized that data was actually stored in a separate hash
table. In fact, many of the functions in this area don't actually need to
return anything, so we can trim some code and hopefully reduce confusion a
bit. Patch attached.

[0] https://postgr.es/m/8F1F1E1D-D17B-4B33-B014-EDBCD15F3F0B%40yesql.se

--
nathan