Deprecate the services_tfa module

Problem/Motivation

In Drupal 7, the services module was the de-facto way for providing REST API for Drupal sites. Drupal 8 provided REST services in core, and as such the presence of services_tfa is now a strange beast as it only provides TFA services for those few sites still using the services module (most probably don't actually need it, and have it only as a leftover of the D7->D8 migration).

The TFA module currently has no way to enforce TFA via the core REST services (see #3374260: Allow TFA authentication through REST routes), and the solution for that issue should also cover the sites using the services module. In any case, the use of the services_tfa module is entirely optional, as it wasn't actually preventing authentication via the services APIs without TFA, but simply providing a way to check it via services.

Proposed resolution

Deprecate the module in the 1.x version, and delete it from the 2.x codebase.