Remove CustomPath's TextOutCustomPath method.

You can't really do anything useful with this in the form it currently
exists; among other problems, there's no way to reread whatever
information might be produced when the path is output.  Work is
underway to replace this with a more useful and more general system of
extensible nodes, but let's start by getting rid of this bit.

Extracted from a larger patch by KaiGai Kohei.

doc/src/sgml/custom-scan.sgml

@@ -125,19 +125,6 @@ Plan *(*PlanCustomPath) (PlannerInfo *root,
     be a <literal>CustomScan</> object, which the callback must allocate and
     initialize.  See <xref linkend="custom-scan-plan"> for more details.
    </para>
-
-   <para>
-<programlisting>
-void (*TextOutCustomPath) (StringInfo str,
-                           const CustomPath *node);
-</programlisting>
-    Generate additional output when <function>nodeToString</> is invoked on
-    this custom path.  This callback is optional.  Since
-    <function>nodeToString</> will automatically dump all fields in the
-    structure that it can see, including <structfield>custom_private</>, this
-    is only useful if the <structname>CustomPath</> is actually embedded in a
-    larger struct containing additional fields.
-   </para>
   </sect2>
  </sect1>
 

src/backend/nodes/outfuncs.c

@@ -1705,8 +1705,6 @@ _outCustomPath(StringInfo str, const CustomPath *node)
 	WRITE_NODE_FIELD(custom_private);
 	appendStringInfoString(str, " :methods ");
 	_outToken(str, node->methods->CustomName);
-	if (node->methods->TextOutCustomPath)
-		node->methods->TextOutCustomPath(str, node);
 }
 
 static void

src/include/nodes/relation.h

@@ -958,9 +958,6 @@ typedef struct CustomPathMethods
 												List *tlist,
 												List *clauses,
 												List *custom_plans);
-	/* Optional: print additional fields besides "private" */
-	void		(*TextOutCustomPath) (StringInfo str,
-											  const struct CustomPath *node);
 } CustomPathMethods;
 
 typedef struct CustomPath