[Cashtab][Alias] Deprecate getPendingAliases

Summary:
T3216

Following the D14211 refactor and deprecation of isAliasAvailable in D14317, getPendingAliases is no longer in use.

Test Plan:
grep -r getPendingAliases src/
npm test

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D14320