Remove `resolved_on_default_branch` method from Vulnerability model

Since we introduced a new attribute called `resolved_on_default_branch`
we can now rely on that attribute instead of calculating this value.

ee/app/models/vulnerability.rb

@@ -141,23 +141,6 @@ class Vulnerability < ApplicationRecord
     true
   end
 
-  def resolved_on_default_branch
-    return false unless findings.any?
-
-    # We can't just use project.latest_successful_pipeline_for_default_branch
-    # because there's no guarantee that it actually ran the security jobs
-    # See https://gitlab.com/gitlab-org/gitlab/-/issues/218012
-    latest_successful_pipeline = project
-      .latest_pipeline_with_security_reports(only_successful: true)
-
-    # Technically this shouldn't ever happen.
-    # If an vulnerability was discovered, then we must have ran a scan of the
-    # appropriate type at least once.
-    return false unless latest_successful_pipeline
-
-    finding.pipelines.exclude?(latest_successful_pipeline)
-  end
-
   def user_notes_count
     user_notes_count_service.count
   end

ee/spec/models/vulnerability_spec.rb

@@ -249,38 +249,6 @@ RSpec.describe Vulnerability do
     it { is_expected.to delegate_method(:name).to(:group).with_prefix.allow_nil }
   end
 
-  describe '#resolved_on_default_branch' do
-    let_it_be(:project) { create(:project, :repository, :with_vulnerability) }
-    let_it_be(:pipeline_with_vulnerability) { create(:ee_ci_pipeline, :success, :with_sast_report, project: project, sha: project.commit.id) }
-    let_it_be(:vulnerability) { project.vulnerabilities.first }
-    let_it_be(:finding1) { create(:vulnerabilities_occurrence, vulnerability: vulnerability, pipelines: [pipeline_with_vulnerability]) }
-    let_it_be(:finding2) { create(:vulnerabilities_occurrence, vulnerability: vulnerability, pipelines: [pipeline_with_vulnerability]) }
-
-    subject { vulnerability.resolved_on_default_branch }
-
-    context 'Vulnerability::Finding is present on the pipeline for default branch' do
-      it { is_expected.to eq(false) }
-
-      context 'but pipeline is failed' do
-        let!(:unsucessful_pipeline_with_vulnerability) { create(:ee_ci_pipeline, :with_sast_report, :failed, project: project, sha: project.commit.id) }
-
-        it { is_expected.to eq(false) }
-      end
-    end
-
-    context 'Vulnerability::Finding is not present on the latest pipeline without security job' do
-      let!(:pipeline_without_security_job) { create(:ee_ci_pipeline, :success, project: project, sha: project.commit.id) }
-
-      it { is_expected.to eq(false) }
-    end
-
-    context 'Vulnerability::Finding is not present on the pipeline for default branch' do
-      let!(:pipeline_without_vulnerability) { create(:ee_ci_pipeline, :success, :with_sast_report, project: project, sha: project.commit.id) }
-
-      it { is_expected.to eq(true) }
-    end
-  end
-
   describe '#resource_parent' do
     let(:vulnerability) { build(:vulnerability) }