Handle unused assignments in use-walrus-if codemod

[drdavella]

Details

In some cases the use-walrus-if codemod makes a change such that the involved variable is no longer used at all. In other words, the original conditional is the only place where the result of the assignment was used.

We can use the scope provider and parent metadata in libCST to determine which such cases occur. In these cases we should not rewrite with the walrus operator at all but simply use the RHS of the original assign expression as the conditional value without any assignment at all.

We can actually see an example where the current behavior causes our own pipelines to fail in #258.

[drdavella]

This is reasonably urgent because it causes pylint jobs to fail when the assignment is no longer used.