stream sidebar: Clean up unsubscribe logic.

From subs.js we don't redundantly try to remove an element
from ths sidebar; we just trigger the event.

In stream_list.js we continue to remove the element from
the DOM, and we also remove the widget from our internal
Dict of sidebar rows, so that if we re-subscribe, we know
we'll automatically re-build the widget from the template
and the latest data from stream_data.js.

static/js/stream_list.js

@@ -51,6 +51,16 @@ exports.stream_sidebar = (function () {
         return self.rows.has(stream_id);
     };
 
+    self.remove_row = function (stream_id) {
+        var widget = self.rows.get(stream_id);
+        if (!widget) {
+            blueslip.warn('Cannot remove stream id ' + stream_id);
+            return;
+        }
+        widget.remove();
+        self.rows.del(stream_id);
+    };
+
     return self;
 }());
 
@@ -249,6 +259,10 @@ function build_stream_sidebar_row(sub) {
         return list_item;
     };
 
+    self.remove = function () {
+        list_item.remove();
+    };
+
     exports.stream_sidebar.set_row(sub.stream_id, self);
 }
 
@@ -348,10 +362,6 @@ exports.set_pm_conversation_count = function (conversation, count) {
     update_count_in_dom(count_span, value_span, count);
 };
 
-exports.remove_narrow_filter = function (name, type) {
-    get_filter_li(type, name).remove();
-};
-
 function rebuild_recent_topics(stream) {
     // TODO: Call rebuild_recent_topics less, not on every new
     // message.
@@ -510,8 +520,7 @@ $(function () {
     });
 
     $(document).on('subscription_remove_done.zulip', function (event) {
-        var stream_name = event.sub.name;
-        exports.remove_narrow_filter(stream_name, 'stream');
+        exports.stream_sidebar.remove_row(event.sub.stream_id);
         // We need to make sure we resort if the removed sub gets added again
         previous_sort_order = undefined;
         previous_unpinned_order = undefined;

static/js/subs.js

@@ -442,8 +442,6 @@ exports.mark_sub_unsubscribed = function (sub) {
         // We don't know about this stream
         return;
     } else if (sub.subscribed) {
-        stream_list.remove_narrow_filter(sub.name, 'stream');
-
         stream_data.unsubscribe_myself(sub);
 
         var button = button_for_sub(sub);