The current implementation cannot handle this correctly, so just
forbid it for now.

GENERATED clauses must be attached to the column definition and cannot
be added later like DEFAULT, so if a child table has a generation
expression that the parent does not have, the child column will
necessarily be an attlocal column.  So to implement ALTER TABLE ONLY /
DROP EXPRESSION, we'd need extra code to update attislocal of the
direct child tables, somewhat similar to how DROP COLUMN does it, so
that the resulting state can be properly dumped and restored.

Discussion: https://www.postgresql.org/message-id/flat/15830.1575468847%40sss.pgh.pa.us