[alias-server] deprecate return of pending alias txs from getValidAliasRegistrations function

Summary:
T3060

Depends on D13518

It doesn't make sense to get pending (i.e. unconfirmed but otherwise valid) alias registrations in a function that will be called when new blocks are found. Pending alias txs will be handled separately.

Test Plan:
npm test
node index.js and app works as normal

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D13519