Remove T_MemoryContext

This is an abstract node that shouldn't have a node tag defined.

Discussion: https://www.postgresql.org/message-id/flat/c1097590-a6a4-486a-64b1-e1f9cc0533ce@enterprisedb.com

Author: petere

src/include/nodes/nodes.h

@@ -284,7 +284,6 @@ typedef enum NodeTag
 	/*
 	 * TAGS FOR MEMORY NODES (memnodes.h)
 	 */
-	T_MemoryContext,
 	T_AllocSetContext,
 	T_SlabContext,
 	T_GenerationContext,