Remove test for legacy address when parsing URL in the GUI

Summary: As per title. This is further cleaning up the GUI from legacy addresses.

Test Plan:
Open bitcoin-qt.

Click file -> open URI -> paste a legacy URI.

Ensure that the URL is rejected.

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3573