Remove instance variable assignment from `gitlab_sign_in` helper

Also removes `Thread.current[:current_user]` assignment from
`gitlab_sign_in_via`

spec/support/login_helpers.rb

@@ -15,14 +15,16 @@ module LoginHelpers
   #   user = create(:user)
   #   gitlab_sign_in(user)
   def gitlab_sign_in(user_or_role, **kwargs)
-    @user =
+    user =
       if user_or_role.is_a?(User)
         user_or_role
       else
         create(user_or_role)
       end
 
-    gitlab_sign_in_with(@user, **kwargs)
+    gitlab_sign_in_with(user, **kwargs)
+
+    user
   end
 
   def gitlab_sign_in_via(provider, user, uid)
@@ -53,8 +55,6 @@ module LoginHelpers
     check 'user_remember_me' if remember
 
     click_button "Sign in"
-
-    Thread.current[:current_user] = user
   end
 
   def mock_auth_hash(provider, uid, email)