compose: Remove a redundant TypingNotifier.stoppedComposing call

Issue #720 is superseded by #1441, in which we'll still clear the
compose box when the send button is tapped. (We'll still preserve
the composing progress in case the send fails, but we'll do so in an
OutboxMessage instead of within the compose input in a disabled
state.)

Author: chrisbobbe

lib/widgets/compose_box.dart

@@ -1288,10 +1288,6 @@ class _SendButtonState extends State<_SendButton> {
     final content = controller.content.textNormalized;
 
     controller.content.clear();
-    // The following `stoppedComposing` call is currently redundant,
-    // because clearing input sends a "typing stopped" notice.
-    // It will be necessary once we resolve #720.
-    store.typingNotifier.stoppedComposing();
 
     try {
       // TODO(#720) clear content input only on success response;