[lib] Remove unused 3rd-party base64 lib

Summary: Resolves ENG-3678. This lib along with the deleted function weren't used anywhere.

Test Plan: Flow, unit tests, CI checks. Confirmed with git grep that no more occurrences are present.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D7997