Unnecessary `conv_expr` for changing cardinality on computed

- EdgeDB Version: 3.3
- EdgeDB CLI Version: 3.5.0+e1ad387
- OS Version: macos 13.5

Schema
```
type Target_ {
multi link latestSource := (
select . y
Please specify an expression in order to convert link 'latestSource' of object type 'default::Target_' to 'single' cardinality:
conv_expr> (SELECT .latestSource LIMIT 1)
```
The expression seems unnecessary since `latestSource` is a computed. This one is obviously easy, and in fact the CLI fills it in for you (!!) but filing in case this would affect other migration changes to computeds or is a hint of a deeper problem.

Reproduce the migration in the issue using the Target_ and Source_ schema, then trace the CLI migration prompt for changing a computed link from multi to single cardinality. Check how computed links are handled before requesting conv_expr. Done means this change no longer asks for an unnecessary conversion expression while other cardinality conversions still behave correctly.

Written by the indexing model from the issue text.