Resolve lint warning by removing @VisibleForTesting for `getUniqueLabel`

The production code already use getUniqueLabel() for APF programs.
It is no longer required, removing the annotation to resolves lint warning.

Test: atest NetworkStackTests
Change-Id: I19e64b685ce502ba2fe8218b58e679ce7280d9eb
diff --git a/src/android/net/apf/BaseApfGenerator.java b/src/android/net/apf/BaseApfGenerator.java
index 2eab5ab..85d5cb5 100644
--- a/src/android/net/apf/BaseApfGenerator.java
+++ b/src/android/net/apf/BaseApfGenerator.java
@@ -22,7 +22,6 @@
 
 import android.annotation.NonNull;
 
-import com.android.internal.annotations.VisibleForTesting;
 import com.android.net.module.util.ByteUtils;
 import com.android.net.module.util.CollectionUtils;
 import com.android.net.module.util.HexDump;
@@ -855,7 +854,6 @@
     /**
      * Return a unique label string.
      */
-    @VisibleForTesting
     public String getUniqueLabel() {
         return "LABEL_" + mLabelCount++;
     }