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/rewrite/rewriteManip.c

@@ -1143,7 +1143,7 @@ replace_rte_variables_mutator(Node *node,
 			/* Found a matching variable, make the substitution */
 			Node	   *newnode;
 
-			newnode = (*context->callback) (var, context);
+			newnode = context->callback(var, context);
 			/* Detect if we are adding a sublink to query */
 			if (!context->inserted_sublink)
 				context->inserted_sublink = checkExprHasSubLink(newnode);