Remove unnecessary code to handle CONSTR_NOTNULL

Commit 14e87ffa5c needlessly added support for CONSTR_NOTNULL entries
to StoreConstraints.  It's dead code, so remove it.

To make the situation regarding constraint creation clearer, change
comments in heap_create_with_catalog, StoreConstraints, MergeAttributes
to explain which types of constraint are used on each.

Author: 何建 (Jian He) <jian.universality@gmail.com>
Discussion: https://postgr.es/m/CACJufxFxzqrCiUNfjJ0tQU+=nKQkQCGtGzUBude=SMOwj5VNjQ@mail.gmail.com

src/backend/catalog/heap.c

@@ -427,7 +427,7 @@ heap_create(const char *relname,
  *		6) AddNewAttributeTuples() is called to register the
  *		   new relation's schema in pg_attribute.
  *
- *		7) StoreConstraints is called ()		- vadim 08/22/97
+ *		7) StoreConstraints() is called			- vadim 08/22/97
  *
  *		8) the relations are closed and the new relation's oid
  *		   is returned.
@@ -1481,7 +1481,7 @@ heap_create_with_catalog(const char *relname,
 	InvokeObjectPostCreateHookArg(RelationRelationId, relid, 0, is_internal);
 
 	/*
-	 * Store any supplied constraints and defaults.
+	 * Store any supplied CHECK constraints and defaults.
 	 *
 	 * NB: this may do a CommandCounterIncrement and rebuild the relcache
 	 * entry, so the relation must be valid and self-consistent at this point.
@@ -2216,7 +2216,7 @@ StoreRelNotNull(Relation rel, const char *nnname, AttrNumber attnum,
 }
 
 /*
- * Store defaults and constraints (passed as a list of CookedConstraint).
+ * Store defaults and CHECK constraints (passed as a list of CookedConstraint).
  *
  * Each CookedConstraint struct is modified to store the new catalog tuple OID.
  *
@@ -2260,13 +2260,6 @@ StoreConstraints(Relation rel, List *cooked_constraints, bool is_internal)
 				numchecks++;
 				break;
 
-			case CONSTR_NOTNULL:
-				con->conoid =
-					StoreRelNotNull(rel, con->name, con->attnum,
-									!con->skip_validation, con->is_local,
-									con->inhcount, con->is_no_inherit);
-				break;
-
 			default:
 				elog(ERROR, "unrecognized constraint type: %d",
 					 (int) con->contype);

src/backend/commands/tablecmds.c

@@ -1007,9 +1007,9 @@ DefineRelation(CreateStmt *stmt, char relkind, Oid ownerId,
 	}
 
 	/*
-	 * Create the relation.  Inherited defaults and constraints are passed in
-	 * for immediate handling --- since they don't need parsing, they can be
-	 * stored immediately.
+	 * Create the relation.  Inherited defaults and CHECK constraints are
+	 * passed in for immediate handling --- since they don't need parsing,
+	 * they can be stored immediately.
 	 */
 	relationId = heap_create_with_catalog(relname,
 										  namespaceId,
@@ -2437,10 +2437,11 @@ storage_name(char c)
  * 'is_partition' tells if the table is a partition.
  *
  * Output arguments:
- * 'supconstr' receives a list of constraints belonging to the parents,
- *		updated as necessary to be valid for the child.
- * 'supnotnulls' receives a list of CookedConstraints that corresponds to
- *		constraints coming from inheritance parents.
+ * 'supconstr' receives a list of CookedConstraint representing
+ *		CHECK constraints belonging to parent relations, updated as
+ *		necessary to be valid for the child.
+ * 'supnotnulls' receives a list of CookedConstraint representing
+ *		not-null constraints based on those from parent relations.
  *
  * Return value:
  * Completed schema list.