chore: remove unused mqtt cap 'subscription_identifiers'

apps/emqx/src/emqx_mqtt_caps.erl

@@ -37,7 +37,6 @@
     max_qos_allowed => emqx_types:qos(),
     retain_available => boolean(),
     wildcard_subscription => boolean(),
-    subscription_identifiers => boolean(),
     shared_subscription => boolean(),
     exclusive_subscription => boolean()
 }.
@@ -66,7 +65,6 @@
     max_qos_allowed,
     retain_available,
     wildcard_subscription,
-    subscription_identifiers,
     shared_subscription,
     exclusive_subscription
 ]).

apps/emqx/test/emqx_channel_SUITE.erl

@@ -275,7 +275,6 @@ t_chan_caps(_) ->
             max_topic_levels := Level,
             retain_available := true,
             shared_subscription := true,
-            subscription_identifiers := true,
             wildcard_subscription := true
         } when is_integer(Level),
         emqx_channel:caps(channel())