pg_dump: Remove some unused return values.

pg_dump: Remove some unused return values.

getSchemaData() does not use the return values of many of its get*
helper functions because they store the data elsewhere.  For
example, commit 92316a4582 introduced a separate hash table for
dumpable objects that said helper functions populate.  This commit
changes these functions to return void and removes their "int *"
parameters that returned the number of objects found.

Reviewed-by: Neil Conway, Tom Lane, Daniel Gustafsson
Discussion: https://postgr.es/m/ZmCAtVaOrHpf31PJ%40nathan