[Cashtab][Alias] Deprecate isAliasServerOnline

Summary:
T3216

As per feedback from D14211, removing the redundant isAliasServerOnline as the newly implemented alias-server wrappers already cater for the non-responsive server and other error scenarios.

Test Plan:
grep -r isAliasServerOnline src/ and ensure no outputs
npm test

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14246