refactor: Remove maybe_clear_subscribers().

The maybe_clear_subscribers() function was an artifact of
when we used to attach subscribers to the "sub" records in
stream_data.js.  I think it was basically a refactoring
shim, and due to some other recent cleanup, it was only
used in test code.

We also change how we validate stream ids.

Going forward, peer_data just looks up stream_ids with the
normal stream_data API when it's trying to warn about
rogue stream_ids coming in.  As I alluded to in an earlier
commit, some of the warning code here might be overly
defensive, but at least it's pretty self-contained.

frontend_tests/node_tests/compose.js

@@ -972,6 +972,8 @@ run_test("warn_if_private_stream_is_linked", () => {
         stream_id: 100,
         name: "Denmark",
     };
+    stream_data.add_sub(denmark);
+
     peer_data.set_subscribers(denmark.stream_id, [1, 2, 3]);
 
     function test_noop_case(invite_only) {

frontend_tests/node_tests/peer_data.js

@@ -240,7 +240,11 @@ run_test("get_subscriber_count", () => {
 
 run_test("is_subscriber_subset", () => {
     function make_sub(stream_id, user_ids) {
-        const sub = {stream_id};
+        const sub = {
+            stream_id,
+            name: `stream ${stream_id}`,
+        };
+        stream_data.add_sub(sub);
         peer_data.set_subscribers(sub.stream_id, user_ids);
         return sub;
     }