UpstreamNetworkMonitor: remove unused mTetheringUpstreamNetwork

The variable mTetheringUpstreamNetwork is unused.
Remove the dead variable and its related setter setCurrentUpstream().

History:
aosp/507635: add mTetheringUpstreamNetwork
aosp/1183410: some functions stop using mTetheringUpstreamNetwork
aosp/1203243: stop using mTetheringUpstreamNetwork completely

Bug: 243516306
Test: atest TetheringTest
Change-Id: I07784c47e49dd9ef034f91bef9089ca48c187f30
diff --git a/Tethering/src/com/android/networkstack/tethering/Tethering.java b/Tethering/src/com/android/networkstack/tethering/Tethering.java
index e48019c..b4e5781 100644
--- a/Tethering/src/com/android/networkstack/tethering/Tethering.java
+++ b/Tethering/src/com/android/networkstack/tethering/Tethering.java
@@ -1856,7 +1856,6 @@
             final Network newUpstream = (ns != null) ? ns.network : null;
             if (mTetherUpstream != newUpstream) {
                 mTetherUpstream = newUpstream;
-                mUpstreamNetworkMonitor.setCurrentUpstream(mTetherUpstream);
                 reportUpstreamChanged(ns);
             }
         }
diff --git a/Tethering/src/com/android/networkstack/tethering/UpstreamNetworkMonitor.java b/Tethering/src/com/android/networkstack/tethering/UpstreamNetworkMonitor.java
index 16c031b..ac2aa7b 100644
--- a/Tethering/src/com/android/networkstack/tethering/UpstreamNetworkMonitor.java
+++ b/Tethering/src/com/android/networkstack/tethering/UpstreamNetworkMonitor.java
@@ -133,8 +133,6 @@
     private boolean mIsDefaultCellularUpstream;
     // The current system default network (not really used yet).
     private Network mDefaultInternetNetwork;
-    // The current upstream network used for tethering.
-    private Network mTetheringUpstreamNetwork;
     private boolean mPreferTestNetworks;
 
     public UpstreamNetworkMonitor(Context ctx, StateMachine tgt, SharedLog log, int what) {
@@ -191,7 +189,6 @@
         releaseCallback(mListenAllCallback);
         mListenAllCallback = null;
 
-        mTetheringUpstreamNetwork = null;
         mNetworkMap.clear();
     }
 
@@ -342,11 +339,6 @@
         return findFirstDunNetwork(mNetworkMap.values());
     }
 
-    /** Tell UpstreamNetworkMonitor which network is the current upstream of tethering. */
-    public void setCurrentUpstream(Network upstream) {
-        mTetheringUpstreamNetwork = upstream;
-    }
-
     /** Return local prefixes. */
     public Set<IpPrefix> getLocalPrefixes() {
         return (Set<IpPrefix>) mLocalPrefixes.clone();