Only handle cashaddr prefix in OpenURIDialog

Summary: This allows for code simplification.

Test Plan:

make src/qt/bitcoin-qt

Run bitcoin-qt on testnet. Use file, open URI and make sure the expected prefix correspond to cashaddr.

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3539