code cleanup: Remove obsolete stream_sort code.

I remove an obsolete comment--we use get_streams()
for the `n` key now.

I also remove a guard statement from sort_groups()
that returned `undefined` for empty lists.

That guard statement would break this code:

    const stream_groups = stream_sort.sort_groups(streams, get_search_term());

    if (stream_groups.same_as_before && ...

The calling code prevents the situation anyway:

    const streams = stream_data.subscribed_stream_ids();
    if (streams.length === 0) {
        return;
    }

I modify the "no_subscribed_streams" test to test
the new behavior.  (Even though stream_list currently
short-circuits the call here, that may change in the future.)

I also introduce the test() wrapper to explicitly clear
our data.

frontend_tests/node_tests/stream_sort.js

@@ -8,11 +8,6 @@ const {run_test} = require("../zjsunit/test");
 const stream_data = zrequire("stream_data");
 const stream_sort = zrequire("stream_sort");
 
-run_test("no_subscribed_streams", () => {
-    assert.equal(stream_sort.sort_groups([]), undefined);
-    assert.equal(stream_sort.first_stream_id(), undefined);
-});
-
 const scalene = {
     subscribed: true,
     name: "scalene",
@@ -44,18 +39,36 @@ const weaving = {
     pin_to_top: false,
 };
 
+function sort_groups(query) {
+    const streams = stream_data.subscribed_stream_ids();
+    return stream_sort.sort_groups(streams, query);
+}
+
+function test(label, f) {
+    run_test(label, (override) => {
+        stream_data.clear_subscriptions();
+        f(override);
+    });
+}
+
+test("no_subscribed_streams", () => {
+    const sorted = sort_groups("");
+    assert.deepEqual(sorted, {
+        dormant_streams: [],
+        normal_streams: [],
+        pinned_streams: [],
+        same_as_before: sorted.same_as_before,
+    });
+    assert.equal(stream_sort.first_stream_id(), undefined);
+});
+
+test("basics", (override) => {
     stream_data.add_sub(scalene);
     stream_data.add_sub(fast_tortoise);
     stream_data.add_sub(pneumonia);
     stream_data.add_sub(clarinet);
     stream_data.add_sub(weaving);
 
-function sort_groups(query) {
-    const streams = stream_data.subscribed_stream_ids();
-    return stream_sort.sort_groups(streams, query);
-}
-
-run_test("basics", (override) => {
     override(stream_data, "is_active", (sub) => sub.name !== "pneumonia");
 
     // Test sorting into categories/alphabetized

static/js/stream_sort.js

@@ -7,8 +7,6 @@ let previous_dormant;
 let all_streams = [];
 
 export function get_streams() {
-    // Right now this is only used for testing, but we should
-    // use it for things like hotkeys that cycle through streams.
     const sorted_streams = all_streams.map((stream_id) =>
         stream_data.maybe_get_stream_name(stream_id),
     );
@@ -46,10 +44,6 @@ function filter_streams_by_search(streams, search_term) {
 }
 
 export function sort_groups(streams, search_term) {
-    if (streams.length === 0) {
-        return undefined;
-    }
-
     streams = filter_streams_by_search(streams, search_term);
 
     function is_normal(sub) {