In commit 8b08f7d4 I added member relationId to IndexStmt struct.
I'm now not sure why; DefineIndex doesn't need it, since the relation
OID is passed as a separate argument anyway.  Remove it.

Also remove a redundant assignment to the relationId argument (it wasn't
redundant when added by commit e093dcdd, but should have been removed
in commit 5f173040), and use relationId instead of stmt->relation when
locking the relation in the second phase of CREATE INDEX CONCURRENTLY,
which is not only confusing but it means we resolve the name twice for
no reason.