phlsys_conduit: remove barely used 'get_user'

Simplify the interface of phlsys_conduit prior to considering supporting
multiple threads.

Remove barely used 'get_user' method, the same information is easily
available elsewhere.

Test Plan:
$ ./precommit.sh
$ git grep get_user
$ arcyon query --mine --uri https://127.0.0.1
WIP: MultiConduit