Revert "wifi: Add systemAPi:setSubscriberId in NetworkAgentConfig"

This reverts commit a37dda602483a59cba903964bc206594116eb04e.

Reason for revert: After discussion, we prefer to add a field in NetworkCapabilities instead. Sorry for inconvenience.

Change-Id: I2e8840a797df9d1594f2b8b576bcd45beb184fca
diff --git a/core/java/android/net/NetworkAgentConfig.java b/core/java/android/net/NetworkAgentConfig.java
index c29d23d..fe1268d 100644
--- a/core/java/android/net/NetworkAgentConfig.java
+++ b/core/java/android/net/NetworkAgentConfig.java
@@ -272,6 +272,7 @@
          * Sets the subscriber ID for this network.
          *
          * @return this builder, to facilitate chaining.
+         * @hide
          */
         @NonNull
         public Builder setSubscriberId(@Nullable String subscriberId) {