Fix Projects::MergeRequests::DiffsController specs

These specs were flaky and only passing after a retry due to how
rspec-retry works.

1. The test with paths that don't exist was returning 200 on the first
  try, then 404 on the second, not because the paths don't exist, but
  because the MR IID didn't change, thus the MR couldn't be found.
  I decided to remove the test entirely since we don't seem to return
  404 for paths that don't exist.
2. The test with a user that cannot view the merge request was failing
  the first time because the project owner wasn't removed with
  `project.team.truncate`.
  Changing the `let(:user)` to `create(:user)` and calling
  `project.add_maintainer(user)` in the `before` block fix the test.
Signed-off-by: NRémy Coutable <remy@rymai.me>

app/controllers/projects/merge_requests/diffs_controller.rb

 # frozen_string_literal: true
 
 class Projects::MergeRequests::DiffsController < Projects::MergeRequests::ApplicationController
-  include DiffForPath
   include DiffHelper
   include RendersNotes
 

spec/controllers/projects/merge_requests/diffs_controller_spec.rb

@@ -4,10 +4,11 @@ describe Projects::MergeRequests::DiffsController do
   include ProjectForksHelper
 
   let(:project) { create(:project, :repository) }
-  let(:user)    { project.owner }
+  let(:user) { create(:user) }
   let(:merge_request) { create(:merge_request_with_diffs, target_project: project, source_project: project) }
 
   before do
+    project.add_maintainer(user)
     sign_in(user)
   end
 
@@ -114,16 +115,6 @@ describe Projects::MergeRequests::DiffsController do
             expect(paths).to include(existing_path)
           end
         end
-
-        context 'when the path does not exist in the diff' do
-          before do
-            diff_for_path(old_path: 'files/ruby/nopen.rb', new_path: 'files/ruby/nopen.rb')
-          end
-
-          it 'returns a 404' do
-            expect(response).to have_gitlab_http_status(404)
-          end
-        end
       end
 
       context 'when the user cannot view the merge request' do