frontend/tko/rpc_interface.py: Remove duplicate function

On commit c2cf6683, the method get_test_labels was
added, when there was already such a method with the
exact same implementation. Let's get rid of the newest.

CC: Cleber Rosa <crosa@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>