refactor(session): drop `lookup/1` session API

Due to the fact it's not used for anything right now.

apps/emqx/src/emqx_persistent_session_ds.erl

@@ -22,14 +22,10 @@
 -include("emqx_mqtt.hrl").
 
 %% Session API
--export([
-    lookup/1,
-    destroy/1
-]).
-
 -export([
     create/3,
-    open/2
+    open/2,
+    destroy/1
 ]).
 
 -export([
@@ -106,10 +102,6 @@ create(#{clientid := ClientID}, _ConnInfo, Conf) ->
 open(#{clientid := ClientID}, _ConnInfo) ->
     open_session(ClientID).
 
--spec lookup(emqx_types:clientinfo()) -> none.
-lookup(_ClientInfo) ->
-    'TODO'.
-
 -spec destroy(session() | clientinfo()) -> ok.
 destroy(#{clientid := ClientID}) ->
     emqx_ds:session_drop(ClientID).

apps/emqx/src/emqx_session.erl

@@ -57,11 +57,7 @@
 -export([
     create/2,
     open/2,
-    destroy/1
-]).
-
--export([
-    lookup/2,
+    destroy/1,
     destroy/2
 ]).
 
@@ -145,8 +141,8 @@
 }.
 
 -type t() ::
-    emqx_session_mem:t()
-    | emqx_session_ds:t().
+    emqx_session_mem:session()
+    | emqx_persistent_session_ds:session().
 
 -define(INFO_KEYS, [
     id,
@@ -198,10 +194,6 @@ get_mqtt_conf(Zone, Key) ->
 %% Existing sessions
 %% -------------------------------------------------------------------
 
--spec lookup(clientinfo(), conninfo()) -> t() | none.
-lookup(ClientInfo, ConnInfo) ->
-    (choose_impl_mod(ConnInfo)):lookup(ClientInfo).
-
 -spec destroy(clientinfo(), conninfo()) -> ok.
 destroy(ClientInfo, ConnInfo) ->
     (choose_impl_mod(ConnInfo)):destroy(ClientInfo).

apps/emqx/src/emqx_session_mem.erl

@@ -55,14 +55,10 @@
 -compile(nowarn_export_all).
 -endif.
 
--export([
-    lookup/1,
-    destroy/1
-]).
-
 -export([
     create/3,
-    open/2
+    open/2,
+    destroy/1
 ]).
 
 -export([
@@ -185,12 +181,6 @@ get_mqtt_conf(Zone, Key) ->
 get_mqtt_conf(Zone, Key, Default) ->
     emqx_config:get_zone_conf(Zone, [mqtt, Key], Default).
 
--spec lookup(emqx_types:clientinfo()) -> none.
-lookup(_ClientInfo) ->
-    % NOTE
-    % This is a stub. This session impl has no backing store, thus always `none`.
-    none.
-
 -spec destroy(session() | clientinfo()) -> ok.
 destroy(_Session) ->
     % NOTE