Arcyd: remove dead code in abdmail_mailer.py

Test Plan:
git grep for method names
$ ./precommit.sh
Arycd: get landing author from latest commit

Unconditionally copy the author's details from the latest commit
on the review branch and use those for the landing commit.

This replaces the 'determine primary Phabricator user from branch' logic
which is less robust.

Add a new test for the 'latest committer is not a user' scenario.

Test Plan: $ ./precommit.sh