memo: release replacer closure during Memo.Detach

In #120875 we added a replacer closure to the memo, so that we could
call back into `norm.Factory.Replace` from within
`statisticsBuilder.factorOutVirtualCols`, which only has access to the
memo and not the factory. This closure, however, means that the memo
could potentially prevent GC of, or have an invalid reference to the
factory after detachment. We shouldn't need to run the statistics
builder on this memo after detaching from the factory (we shouldn't be
constructing any new expressions in it) so let's go ahead and release
the reference.

Epic: None

Release note: None

Author: michae2

pkg/sql/opt/memo/memo.go

@@ -628,6 +628,8 @@ func (m *Memo) NextRoutineResultBufferID() RoutineResultBufferID {
 // constructed in this memo.
 func (m *Memo) Detach() {
 	m.interner = interner{}
+	m.replacer = nil
+
 	// It is important to not hold on to the EvalCtx in the logicalPropsBuilder
 	// (#57059).
 	m.logPropsBuilder = logicalPropsBuilder{}