Fix thinko in join removal.

Fix thinko in join removal.

In commit 9df8f903e I (tgl) switched join_is_removable() from
using the min relid sets of the join under consideration to
using its full syntactic relid sets.  This was a mistake,
as it allowed join removal in cases where a reference to the
join output would survive in some syntactically-lower join
condition.  Revert to the former coding.

Richard Guo

Discussion: https://postgr.es/m/CAMbWs4-EU9uBGSP7G-iTwLBhRQ=rnZKvFDhD+n+xhajokyPCKg@mail.gmail.com