Make ALTER .. SET SCHEMA do nothing, instead of throwing an ERROR.

Make ALTER .. SET SCHEMA do nothing, instead of throwing an ERROR.

This was already true for CREATE EXTENSION, but historically has not
been true for other object types.  Therefore, this is a backward
incompatibility.  Per discussion on pgsql-hackers, everyone seems to
agree that the new behavior is better.

Marti Raudsepp, reviewed by Haribabu Kommi and myself