Fix return value of rabbit_priority_queue:delete_crashed/1

According to the `rabbit_backing_queue` behavious it must always
return `ok`, but it used to return a list of results one for each
priority. That caused the below crash further up the call chain.

```
> rabbit_classic_queue:delete_crashed(Q)
** exception error: no case clause matching [ok,ok,ok,ok,ok,ok,ok,ok,ok,ok,ok]
     in function  rabbit_classic_queue:delete_crashed/2 (rabbit_classic_queue.erl, line 516)
```

Other backing_queue implementations (`rabbit_variable_queue`) just
exit with a badmatch upon error.

This (very minor) issue is present since 3.13.0 when
`rabbit_classic_queue:delete_crashed_in_backing_queue/1` was
instroduced with Khepri in commit 5f0981c. Before that the result of
`BQ:delete_crashed/1` was simply ignored.

deps/rabbit/src/rabbit_priority_queue.erl

@@ -187,7 +187,9 @@ delete_crashed(Q) ->
     BQ = bq(),
     case priorities(Q) of
         none -> BQ:delete_crashed(Q);
-        Ps   -> [BQ:delete_crashed(mutate_name(P, Q)) || P <- Ps]
+        Ps   ->
+            [ok = BQ:delete_crashed(mutate_name(P, Q)) || P <- Ps],
+            ok
     end.
 
 purge(State = #state{bq = BQ}) ->