remote_mirror: Drop `ensure_remote!` function

The `ensure_remote!` function is not used anymore given that we now
always use in-memory remotes. Remove it.

app/models/remote_mirror.rb

@@ -227,15 +227,6 @@ class RemoteMirror < ApplicationRecord
     Gitlab::UrlSanitizer.new(read_attribute(:url)).full_url
   end
 
-  def ensure_remote!
-    return unless project
-    return unless remote_name && remote_url
-
-    # If this fails or the remote already exists, we won't know due to
-    # https://gitlab.com/gitlab-org/gitaly/issues/1317
-    project.repository.add_remote(remote_name, remote_url)
-  end
-
   def after_sent_notification
     update_column(:error_notification_sent, true)
   end

spec/models/remote_mirror_spec.rb

@@ -388,30 +388,6 @@ RSpec.describe RemoteMirror, :mailer do
     end
   end
 
-  describe '#ensure_remote!' do
-    let(:remote_mirror) { create(:project, :repository, :remote_mirror).remote_mirrors.first }
-    let(:project) { remote_mirror.project }
-    let(:repository) { project.repository }
-
-    it 'adds a remote multiple times with no errors' do
-      expect(repository).to receive(:add_remote).with(remote_mirror.remote_name, remote_mirror.url).twice.and_call_original
-
-      2.times do
-        remote_mirror.ensure_remote!
-      end
-    end
-
-    context 'SSH public-key authentication' do
-      it 'omits the password from the URL' do
-        remote_mirror.update!(auth_method: 'ssh_public_key', url: 'ssh://git:pass@example.com')
-
-        expect(repository).to receive(:add_remote).with(remote_mirror.remote_name, 'ssh://git@example.com')
-
-        remote_mirror.ensure_remote!
-      end
-    end
-  end
-
   describe '#url=' do
     let(:remote_mirror) { create(:project, :repository, :remote_mirror).remote_mirrors.first }
 

spec/services/projects/update_remote_mirror_service_spec.rb

@@ -24,12 +24,6 @@ RSpec.describe Projects::UpdateRemoteMirrorService do
         .and_return(double(divergent_refs: []))
     end
 
-    it 'does not ensure the remote exists' do
-      expect(remote_mirror).not_to receive(:ensure_remote!)
-
-      execute!
-    end
-
     it 'does not fetch the remote repository' do
       # See https://gitlab.com/gitlab-org/gitaly/-/issues/2670
       expect(project.repository).not_to receive(:fetch_remote)