Reduce excessive dereferencing of function pointers

It is equivalent in ANSI C to write (*funcptr) () and funcptr().  These
two styles have been applied inconsistently.  After discussion, we'll
use the more verbose style for plain function pointer variables, to make
it clear that it's a variable, and the shorter style when the function
pointer is in a struct (s.func() or s->func()), because then it's clear
that it's not a plain function name, and otherwise the excessive
punctuation makes some of those invocations hard to read.

Discussion: https://www.postgresql.org/message-id/f52c16db-14ed-757d-4b48-7ef360b1631d@2ndquadrant.com

src/backend/executor/execParallel.c

@@ -1081,5 +1081,5 @@ ParallelQueryMain(dsm_segment *seg, shm_toc *toc)
 	/* Cleanup. */
 	dsa_detach(area);
 	FreeQueryDesc(queryDesc);
-	(*receiver->rDestroy) (receiver);
+	receiver->rDestroy(receiver);
 }