repository: Drop `#with_config` function

The `#with_config` function is not used anymore, and Gitaly has
deprecated direct access to the gitconfig in favor of more specialized
RPCs. Remove the function.

ee/app/models/ee/repository.rb

@@ -17,15 +17,6 @@ module EE
       delegate :checksum, :find_remote_root_ref, to: :raw_repository
     end
 
-    # Transiently sets a configuration variable
-    def with_config(values = {})
-      raw_repository.set_config(values)
-
-      yield
-    ensure
-      raw_repository.delete_config(*values.keys)
-    end
-
     # Runs code after a repository has been synced.
     def after_sync
       expire_all_method_caches

ee/spec/models/repository_spec.rb

@@ -62,35 +62,6 @@ RSpec.describe Repository do
     end
   end
 
-  describe '#with_config' do
-    let(:rugged) { rugged_repo(repository) }
-    let(:entries) do
-      {
-        'test.foo1' => 'hello',
-        'test.foo2' => 'world',
-        'http.http://gitlab-primary.geo/gitlab-qa-sandbox-group/qa-test-10-07-2018-07-22-41/geo-project-ac55ec2cd134afea.wiki.git.extraHeader' => 'Authorization: blabla'
-      }
-    end
-
-    it 'sets config only during the block' do
-      keys_should_not_be_set
-
-      repository.with_config(entries) do
-        entries.each do |key, value|
-          expect(rugged.config[key]).to eq(value)
-        end
-      end
-
-      keys_should_not_be_set
-    end
-
-    def keys_should_not_be_set
-      entries.each do |key, value|
-        expect(rugged.config[key]).to be_blank
-      end
-    end
-  end
-
   describe "Elastic search", :elastic do
     before do
       stub_ee_application_setting(elasticsearch_search: true, elasticsearch_indexing: true)

ee/spec/services/geo/wiki_sync_service_spec.rb

@@ -37,7 +37,6 @@ RSpec.describe Geo::WikiSyncService, :geo do
     include_context 'lease handling'
 
     it 'fetches wiki repository with JWT credentials' do
-      expect(repository).to receive(:with_config).with("http.#{url_to_repo}.extraHeader" => anything).and_call_original
       expect(repository).to receive(:fetch_as_mirror)
         .with(url_to_repo, forced: true, http_authorization_header: anything)
         .once