compose_banner: Remove uploads banners when clearing compose box.

Upload banners were not cleared after closing compose box, which meant
that they would remain present in a paused state after compose was reopened.

https://chat.zulip.org/#narrow/stream/9-issues/topic/Incomplete.20Upload.20banner.20remains.20on.20closing.20compose/near/1582602

web/tests/lib/compose_banner.js

@@ -12,6 +12,7 @@ exports.mock_banners = () => {
     }
     $("#compose_banners .warning").remove = () => {};
     $("#compose_banners .error").remove = () => {};
+    $("#compose_banners .upload_banner").remove = () => {};
 
     const $stub = $.create("stub_to_remove");
     const $cb = $("#compose_banners");