Remove unnecessary override of `build_comparer` method

This method is already implemented by the superclass which is identical
so we don't need to override this method in subclass.

ee/app/services/ci/compare_security_reports_service.rb

@@ -13,9 +13,5 @@ module Ci
     def get_report(pipeline)
       Security::PipelineVulnerabilitiesFinder.new(pipeline: pipeline, params: { report_type: [params[:report_type]], scope: 'all' }).execute
     end
-
-    def build_comparer(base_pipeline, head_pipeline)
-      comparer_class.new(get_report(base_pipeline), get_report(head_pipeline))
-    end
   end
 end

ee/spec/services/ci/compare_security_reports_service_spec.rb

@@ -82,16 +82,6 @@ RSpec.describe Ci::CompareSecurityReportsService do
         expect(result[:status_reason]).to include('JSON parsing failed')
       end
     end
-
-    describe "#build_comparer" do
-      context "when the head_pipeline is nil" do
-        subject { service.build_comparer(base_pipeline, nil) }
-
-        let(:base_pipeline) { create(:ee_ci_pipeline) }
-
-        specify { expect { subject }.not_to raise_error }
-      end
-    end
   end
 
   describe '#execute CS' do