Revert structural changes to not-null constraints

There are some problems with the new way to handle these constraints
that were detected at the last minute, and require fixes that appear too
invasive to be doing this late in the cycle.  Revert this (again) for
now, we'll try again with these problems fixed.

The following commits are reverted:

    b0e96f3119  Catalog not-null constraints
    9b581c5341  Disallow changing NO INHERIT status of a not-null constraint
    d0ec2ddbe0  Fix not-null constraint test
    ac22a9545c  Move privilege check to the right place
    b0f7dd915b  Check stack depth in new recursive functions
    3af7217942  Update information_schema definition for not-null constraints
    c3709100be  Fix propagating attnotnull in multiple inheritance
    d9f686a72e  Fix restore of not-null constraints with inheritance
    d72d32f52d  Don't try to assign smart names to constraints
    0cd711271d  Better handle indirect constraint drops
    13daa33fa5  Disallow NO INHERIT not-null constraints on partitioned tables
    d45597f72f  Disallow direct change of NO INHERIT of not-null constraints
    21ac38f498  Fix inconsistencies in error messages

Discussion: https://postgr.es/m/202405110940.joxlqcx4dogd@alvherre.pgsql

src/test/regress/sql/alter_table.sql

@@ -854,6 +854,7 @@ alter table atacc1 add constraint "atacc1_pkey" primary key (test);
 alter table atacc1 alter column test drop not null;
 \d atacc1
 alter table atacc1 drop constraint "atacc1_pkey";
+alter table atacc1 alter column test drop not null;
 \d atacc1
 insert into atacc1 values (null);
 alter table atacc1 alter test set not null;
@@ -919,6 +920,14 @@ insert into parent values (NULL);
 insert into child (a, b) values (NULL, 'foo');
 alter table only parent alter a set not null;
 alter table child alter a set not null;
+delete from parent;
+alter table only parent alter a set not null;
+insert into parent values (NULL);
+alter table child alter a set not null;
+insert into child (a, b) values (NULL, 'foo');
+delete from child;
+alter table child alter a set not null;
+insert into child (a, b) values (NULL, 'foo');
 drop table child;
 drop table parent;
 
@@ -2340,9 +2349,6 @@ CREATE TABLE atnotnull1 ();
 ALTER TABLE atnotnull1
   ADD COLUMN a INT,
   ALTER a SET NOT NULL;
-ALTER TABLE atnotnull1
-  ADD COLUMN b INT,
-  ADD NOT NULL b;
 ALTER TABLE atnotnull1
   ADD COLUMN c INT,
   ADD PRIMARY KEY (c);