Fix javascript error with deleting messages in issue or merge request

Elements searched for by jQuery no longer existed in the DOM.
Javascript was also unnecessarily complex, for using jQuery
Fixes #8109

app/assets/javascripts/notes.js.coffee

@@ -355,7 +355,7 @@ class @Notes
     $('.note[id="' + note_id + '"]').each ->
       note = $(this)
       notes = note.closest(".notes")
-      count = notes.closest(".notes_holder").find(".discussion-notes-count")
+      count = notes.closest(".issuable-details").find(".notes-tab .badge")
 
       # check if this is the last note for this line
       if notes.find(".note").length is 1
@@ -365,9 +365,10 @@ class @Notes
 
         # for diff lines
         notes.closest("tr").remove()
-      else
-        # update notes count
-        count.get(0).lastChild.nodeValue = " #{notes.children().length - 1}"
+
+      # update notes count
+      oldNum = parseInt(count.text())
+      count.text(oldNum - 1)
 
       note.remove()