Remove code related to object hierarchy in MySQL

These are not required because MySQL is not
supported anymore

spec/spec_helper.rb

@@ -258,14 +258,6 @@ RSpec.configure do |config|
     Gitlab::CurrentSettings.clear_in_memory_application_settings!
   end
-  config.around(:each, :nested_groups) do |example|
-    example.run if Group.supports_nested_objects?
-  end
-
-  config.around(:each, :postgresql) do |example|
-    example.run if Gitlab::Database.postgresql?
-  end
-
   # This makes sure the `ApplicationController#can?` method is stubbed with the
   # original implementation for all view specs.
   config.before(:each, type: :view) do

spec/support/controllers/githubish_import_controller_shared_examples.rb

@@ -321,7 +321,7 @@ shared_examples 'a GitHub-ish import controller: POST create' do
       end
     end
-    context 'user has chosen an existing nested namespace and name for the project', :postgresql do
+    context 'user has chosen an existing nested namespace and name for the project' do
       let(:parent_namespace) { create(:group, name: 'foo') }
       let(:nested_namespace) { create(:group, name: 'bar', parent: parent_namespace) }
       let(:test_name) { 'test_name' }
@@ -340,7 +340,7 @@ shared_examples 'a GitHub-ish import controller: POST create' do
       end
     end
-    context 'user has chosen a non-existent nested namespaces and name for the project', :postgresql do
+    context 'user has chosen a non-existent nested namespaces and name for the project' do
       let(:test_name) { 'test_name' }
       it 'takes the selected namespace and name' do
@@ -371,7 +371,7 @@ shared_examples 'a GitHub-ish import controller: POST create' do
       end
     end
-    context 'user has chosen existent and non-existent nested namespaces and name for the project', :postgresql do
+    context 'user has chosen existent and non-existent nested namespaces and name for the project' do
       let(:test_name) { 'test_name' }
       let!(:parent_namespace) { create(:group, name: 'foo') }

spec/support/shared_contexts/policies/group_policy_shared_context.rb

@@ -31,7 +31,7 @@ RSpec.shared_context 'GroupPolicy context' do
       :admin_group_member,
       :change_visibility_level,
       :set_note_created_at,
-      (Gitlab::Database.postgresql? ? :create_subgroup : nil)
+      :create_subgroup
     ].compact
   end

spec/views/groups/edit.html.haml_spec.rb

@@ -35,7 +35,7 @@ describe 'groups/edit.html.haml' do
       it_behaves_like '"Share with group lock" setting', { disabled: false, checked: false }
     end
-    context 'for a subgroup', :nested_groups do
+    context 'for a subgroup' do
       let!(:subgroup) { create(:group, parent: root_group) }
       let(:sub_owner) { create(:user) }
       let(:test_group) { subgroup }

spec/views/layouts/header/_new_dropdown.haml_spec.rb

@@ -28,7 +28,7 @@ describe 'layouts/header/_new_dropdown' do
         )
       end
-      it 'has a "New subgroup" link', :nested_groups do
+      it 'has a "New subgroup" link' do
         render
         expect(rendered).to have_link(