pgsql: Remove transformAlterTableStmt's kluge to replace

Log Message:
-----------
Remove transformAlterTableStmt's kluge to replace ColumnDef.is_not_null
flags by separate AT_SetNotNull subcommands.  That was always ugly and
inefficient, and it's now clear that it was merely a partial workaround
for the bug just identified in ATExecAddColumn.  This is just code
beautification not a bug fix, so no back-patch.

Brendan Jurd, with some trivial additional cleanup by me.

Modified Files:
--------------
    pgsql/src/backend/parser:
        parse_utilcmd.c (r2.11 -> r2.12)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_utilcmd.c?r1=2.11&r2=2.12)