Remove FIELD_MOBILE_NETWORK in proto.

When user manually selects a cellular network, don't record network
name in metrics logs.

Bug: 65412783
Test: manual
Change-Id: I54b827617f706bfde3615a9fbb2f58939fa74717
diff --git a/proto/src/metrics_constants.proto b/proto/src/metrics_constants.proto
index b6ac3fb..0066193 100644
--- a/proto/src/metrics_constants.proto
+++ b/proto/src/metrics_constants.proto
@@ -4779,7 +4779,7 @@
     // CATEGORY: SETTINGS
     ACTION_MOBILE_NETWORK_MANUAL_SELECT_NETWORK = 1210;
 
-    // FIELD - Manually selected mobile network
+    // Not used anymore.
     FIELD_MOBILE_NETWORK = 1211;
 
     // OPEN: Settings > Network & Internet > Mobile network > Access Point Names