Remove pre-maintenance_mode_status compatibility code

Maintenance mode, introduced in RabbitMQ 3.8.x, was a breaking change
protected behind a feature flag. This allowed a RabbitMQ cluster to be
upgraded one node at a time, without having to stop the entire cluster.

The compatibility code is in the wild for long enough. The
`maintenance_mode_status` feature flag was marked as required in a
previous commit (see #5202). This allows us to remove code in this
patch.

References #5215.

deps/rabbit/src/rabbit_maintenance.erl

@@ -34,7 +34,6 @@
 ]).
 
 -define(TABLE, rabbit_node_maintenance_states).
--define(FEATURE_FLAG, maintenance_mode_status).
 -define(DEFAULT_STATUS,  regular).
 -define(DRAINING_STATUS, draining).
 
@@ -55,26 +54,18 @@
         {requires,    networking}]}).
 
 boot() ->
-    case rabbit_feature_flags:is_enabled(?FEATURE_FLAG, non_blocking) of
-        true ->
-            TableName = status_table_name(),
-            rabbit_log:info(
-                "Creating table ~s for feature flag `~s`",
-                [TableName, ?FEATURE_FLAG]),
-            try
-                _ = rabbit_table:create(
-                    TableName,
-                    status_table_definition())
-            catch throw:Reason  ->
-                rabbit_log:error(
-                    "Failed to create maintenance status table: ~p",
-                    [Reason])
-            end;
-        false ->
-            ok;
-        state_changing ->
-            %% feature flag migration will do the job for us
-            ok
+    TableName = status_table_name(),
+    rabbit_log:info(
+      "Creating table ~s for maintenance mode status",
+      [TableName]),
+    try
+        _ = rabbit_table:create(
+              TableName,
+              status_table_definition())
+    catch throw:Reason  ->
+              rabbit_log:error(
+                "Failed to create maintenance status table: ~p",
+                [Reason])
     end.
 
 %%
@@ -94,19 +85,10 @@ status_table_definition() ->
 
 -spec is_enabled() -> boolean().
 is_enabled() ->
-    rabbit_feature_flags:is_enabled(?FEATURE_FLAG).
+    true.
 
 -spec drain() -> ok.
 drain() ->
-    case is_enabled() of
-        true  -> do_drain();
-        false ->
-            rabbit_log:error("Feature flag '~s' is not enabled, cannot put this node under maintenance", [?FEATURE_FLAG]),
-            {error, rabbit_misc:format("Feature flag '~s' is not enabled, cannot put this node under maintenance", [?FEATURE_FLAG])}
-    end.
-
--spec do_drain() -> ok.
-do_drain() ->
     rabbit_log:warning("This node is being put into maintenance (drain) mode"),
     mark_as_being_drained(),
     rabbit_log:info("Marked this node as undergoing maintenance"),
@@ -135,15 +117,6 @@ do_drain() ->
 
 -spec revive() -> ok.
 revive() ->
-    case is_enabled() of
-        true  -> do_revive();
-        false ->
-            rabbit_log:error("Feature flag '~s' is not enabled, cannot put this node out of maintenance", [?FEATURE_FLAG]),
-            {error, rabbit_misc:format("Feature flag '~s' is not enabled, cannot put this node out of maintenance", [?FEATURE_FLAG])}
-    end.
-
--spec do_revive() -> ok.
-do_revive() ->
     rabbit_log:info("This node is being revived from maintenance (drain) mode"),
     revive_local_quorum_queue_replicas(),
     rabbit_log:info("Resumed all listeners and will accept client connections again"),