Computation of note sk in owns is repeated

[moCello]

Summary

Call gen_note_sk in owns method when checking the ownership.

Detailed Description

From the audit:

In 'phoenix/core/src/keys/secret.rs', the method owns computes whether or not a given note public key corresponds to an owned private key. The first 3 lines compute the note secret key, which are also implemented in the method gen_note_sk. It would be advised to not repeat code and instead use the gen_note_sk method to compute the note secret key, and then retrieving the value out of the generated struct for further computation. This will increase readability and reduce potential error if this logic needs changing or fixing.