Remove extraneous code that was previously added for performance.

app/models/concerns/issuable.rb

@@ -29,8 +29,6 @@ module Issuable
       end
     end
-    has_many :system_notes, -> { merge(Note.system) }, class_name: Note, as: :noteable, inverse_of: :noteable
-
     has_many :label_links, as: :target, dependent: :destroy
     has_many :labels, through: :label_links
     has_many :todos, as: :target, dependent: :destroy

app/models/issue.rb

@@ -201,8 +201,8 @@ class Issue < ActiveRecord::Base
   # From all notes on this issue, we'll select the system notes about linked
   # merge requests. Of those, the MRs closing `self` are returned.
-  def closed_by_merge_requests(current_user = nil, check_if_open: true)
-    return [] if !open? && check_if_open
+  def closed_by_merge_requests(current_user = nil)
+    return [] if !open?
     ext = all_references(current_user)