[Cashtab][Alias] Deprecate isAliasRegistered

Summary:
T3216

Depends on D14317

Following the D14211 refactor, isAliasRegistered is no longer in use.

Test Plan:
grep -r isAliasRegistered src/
npm test
Enable alias flag in Ticker.js
npm start
navigate to the Configure screen and ensure successful adding of contact using a valid alias and an error for invalid aliases (unregistered aliases will be handled in a separate diff - see T3216)

Reviewers: bytesofman, #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14318