DEV: removes unused group_manager model (#22827)

The associated table has been removed in 2015: 6dd4bc7d57 (diff-53b8234c51c429b92eb91d0212e15bbab16fcd5d1cbd3db64509977c9e1a060d)

app/models/group_manager.rb

@@ -1,21 +0,0 @@
-# frozen_string_literal: true
-
-class GroupManager < ActiveRecord::Base
-  belongs_to :group
-  belongs_to :manager, class_name: "User", foreign_key: :user_id
-end
-
-# == Schema Information
-#
-# Table name: group_managers
-#
-#  id         :integer          not null, primary key
-#  group_id   :integer          not null
-#  user_id    :integer          not null
-#  created_at :datetime
-#  updated_at :datetime
-#
-# Indexes
-#
-#  index_group_managers_on_group_id_and_user_id  (group_id,user_id) UNIQUE
-#