[Cashtab] Do not specify change address in getSlpGenesisTargetOutput

Summary:
Standard API in ecash-coinselect is to create change outputs with no specified address. So, Cashtab already handles these outputs by adding Cashtab's expected change address, which is the Path1899 address.

We should not be hardcoding this into only 1 of our many specialty tx building functions. It should follow this existing standard.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D15958