[alias-server] Deprecate isRegistered: true

Summary:
If you have a blockheight and a txid, isRegistered: true is redundant.

Preserving isRegistered: false for aliases that are not registered. This is useful information, esp as you can only get it if you query a valid alias.

The meaning of a returned object with just {alias} is not intuitively understandable.

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14251