Remove redundant method delegation on group model

This change also cleans up some code styles on related specs according
to Rubocop todos.

app/models/group.rb

@@ -118,8 +118,6 @@ class Group < Namespace
       .where("project_authorizations.user_id IN (?)", user_ids)
   end
 
-  delegate :default_branch_name, to: :namespace_settings
-
   class << self
     def sort_by_attribute(method)
       if method == 'storage_size_desc'

spec/models/group_spec.rb

@@ -2214,17 +2214,17 @@ RSpec.describe Group do
   end
 
   describe "#default_branch_name" do
-    context "group.namespace_settings does not have a default branch name" do
+    context "when group.namespace_settings does not have a default branch name" do
       it "returns nil" do
         expect(group.default_branch_name).to be_nil
       end
     end
 
-    context "group.namespace_settings has a default branch name" do
+    context "when group.namespace_settings has a default branch name" do
       let(:example_branch_name) { "example_branch_name" }
 
       before do
-        expect(group.namespace_settings)
+        allow(group.namespace_settings)
           .to receive(:default_branch_name)
           .and_return(example_branch_name)
       end