Only generate cashaddr URI from the GUI

Summary: This is a first step toward deprecating the legacy address format. The Qt interface is not widely used and we can simplify quite a bit of code doing so.

Test Plan: Updated the test cases for URI generation.

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3536