Simplify createDerivedStore in debug.derived.ts

[dskloetd]

Motivation

Small optimization.
If all we want is a Readable view on a writable store, that doesn't expose set and update, creating a derived store is overkill.
The derived store subscribes to the original and forwards the changes but this is not necessary.

Changes

Change createDerivedStore to simply return an object with the same subscribe method and no other methods.

Tests

Refactoring should be covered by existing tests.

[lmuntaner]

We don't have tests for this feature.

Did you try that it worked by:

• Go to Wallet page.
• Click six times in the header.
• Enter "fo" in the prompt.
• Does the JSON file have the data of the "selectedAccount"?

[dskloetd]

Actually, I forgot to bind the method to the object so I expect it won't work.
I'll pause this until I can add a test that would have caught it.

[anedos-dfinity]

Should this PR be closed ?

[lmuntaner]

@dskloetd what do you think?

[dskloetd]

Does it matter? It's marked as Draft already.