Update the phoenix version and starting migration the project.

lib/link_shortener_web/auth/error_handler.ex

@@ -1,10 +0,0 @@
-defmodule LinkShortenerWeb.Auth.ErrorHandler do
-  import Plug.Conn
-
-  def auth_error(conn, {type, _reason}, _opts) do
-    body = Poison.encode!(%{error: to_string(type)})
-    conn
-    |> put_resp_content_type("application/json")
-    |> send_resp(401, body)
-  end
-end

lib/link_shortener_web/auth/guardian.ex

@@ -1,31 +0,0 @@
-defmodule LinkShortenerWeb.Auth.Guardian do
-  use Guardian, otp_app: :link_shortener
-
-  alias LinkShortener.Accounts
-  alias LinkShortener.Accounts.User
-
-  def subject_for_token(user, _claims) do
-    sub = to_string(user.id)
-    {:ok, sub}
-  end
-
-  def resource_from_claims(claims) do
-    id = claims["sub"]
-    resource = Accounts.get_user!(id)
-    {:ok,  resource}
-  end
-
-  def authenticate(email, password) do
-    with user <- Accounts.get_user_by_email_and_password(email, password) do
-      case user do
-        %User{} -> create_token(user)
-        nil -> {:error, :unauthorized}
-      end
-    end
-  end
-
-  defp create_token(user) do
-    {:ok, token, _claims} = encode_and_sign(user)
-    {:ok, user, token}
-  end
-end

lib/link_shortener_web/auth/pipeline.ex

@@ -1,9 +0,0 @@
-defmodule LinkShortenerWeb.Auth.Pipeline do
-  use Guardian.Plug.Pipeline, otp_app: :link_shortener,
-    module: LinkShortenerWeb.Auth.Guardian,
-    error_handler: LinkShortenerWeb.Auth.ErrorHandler
-
-  plug Guardian.Plug.VerifyHeader
-  plug Guardian.Plug.EnsureAuthenticated
-  plug Guardian.Plug.LoadResource
-end