Remove transformAlterTableStmt's kluge to replace ColumnDef.is_not_null

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.