[Cashtab][Alias] Deprecate isAliasAvailable

Summary:
T3216

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

Test Plan:
grep -r isAliasAvailable src/
npm test

Reviewers: bytesofman, #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14317