Remove more (void) and fix -Wall warnings.

src/backend/access/gist/gist.c

src/backend/access/hash/hashinsert.c

src/backend/access/hash/hashovfl.c

src/backend/access/hash/hashpage.c

src/backend/access/heap/heapam.c

src/backend/access/heap/stats.c

src/backend/access/index/indexam.c

src/backend/access/nbtree/nbtsearch.c

src/backend/access/nbtree/nbtsort.c

src/backend/access/rtree/rtree.c

src/backend/access/transam/transsup.c

src/backend/access/transam/varsup.c

src/backend/access/transam/xact.c

src/backend/access/transam/xid.c

src/backend/bootstrap/bootstrap.c

src/backend/catalog/heap.c

src/backend/catalog/index.c

src/backend/catalog/pg_operator.c

src/backend/catalog/pg_type.c

src/backend/commands/async.c