Remove unused Namespace#all_pipelines

This is related to some work where we are looking into moving some
tables to a separate logical database. This query may be a little tricky
to do if we move `ci_pipelines` to a separate logical database.

However, it appears this method is unused anyway so we may as well just
remove it for now.

app/models/namespace.rb

@@ -275,12 +275,6 @@ class Namespace < ApplicationRecord
     Project.where(namespace: namespace)
   end
 
-  # Includes pipelines from this namespace and pipelines from all subgroups
-  # that belongs to this namespace
-  def all_pipelines
-    Ci::Pipeline.where(project: all_projects)
-  end
-
   def has_parent?
     parent_id.present? || parent.present?
   end

spec/models/namespace_spec.rb

@@ -1042,17 +1042,6 @@ RSpec.describe Namespace do
     end
   end
 
-  describe '#all_pipelines' do
-    let(:group) { create(:group) }
-    let(:child) { create(:group, parent: group) }
-    let!(:project1) { create(:project_empty_repo, namespace: group) }
-    let!(:project2) { create(:project_empty_repo, namespace: child) }
-    let!(:pipeline1) { create(:ci_empty_pipeline, project: project1) }
-    let!(:pipeline2) { create(:ci_empty_pipeline, project: project2) }
-
-    it { expect(group.all_pipelines.to_a).to match_array([pipeline1, pipeline2]) }
-  end
-
   describe '#share_with_group_lock with subgroups' do
     context 'when creating a subgroup' do
       let(:subgroup) { create(:group, parent: root_group )}