worker(alarms): remove dead FailoverAlarmConsumer.subscriptionExpression

B4: the field was stored in Subscribe but never read — the primary is never
re-subscribed during probing. Drop it and keep the rationale as a comment.

src/ZB.MOM.WW.MxGateway.Worker/MxAccess/FailoverAlarmConsumer.cs

@@ -65,13 +65,6 @@ public sealed class FailoverAlarmConsumer : IMxAccessAlarmConsumer
     private bool disposed;
     private DateTime lastProbeAtUtc = DateTime.MinValue;
 
-    /// <summary>
-    ///     The subscription expression passed to <see cref="Subscribe"/>.
-    ///     Stored for documentation and potential future full re-subscribe
-    ///     scenarios; the primary is NOT re-subscribed during probing.
-    /// </summary>
-    private string subscriptionExpression = string.Empty;
-
     /// <summary>
     ///     Composes the failover consumer over its two children.
     /// </summary>
@@ -119,9 +112,9 @@ public sealed class FailoverAlarmConsumer : IMxAccessAlarmConsumer
     {
         if (disposed) throw new ObjectDisposedException(nameof(FailoverAlarmConsumer));
 
-        // Store for documentation; the primary is not torn down on failover
+        // The primary is not torn down on failover and is therefore never
-        // and is therefore not re-subscribed during ProbeOnce.
+        // re-subscribed during ProbeOnce, so the subscription expression does
-        subscriptionExpression = subscription;
+        // not need to be retained here.
 
         // Arm the standby first so it is warm regardless of primary outcome.
         // A standby subscribe failure is a hard fault (the fallback itself is