Remove an unused RPC

This RPC has been replaced by the RawPatch RPC, which is called by
workhorse as creating a patch could take a long time. The CommitPatch
RPC has no callsites in this codebase, which allows us to remove it.

Part of: https://gitlab.com/gitlab-org/gitaly/issues/2095

lib/gitlab/gitaly_client/commit_service.rb

@@ -298,18 +298,6 @@ module Gitlab
         Gitlab::SafeRequestStore[key] = commit
       end
 
-      # rubocop: disable CodeReuse/ActiveRecord
-      def patch(revision)
-        request = Gitaly::CommitPatchRequest.new(
-          repository: @gitaly_repo,
-          revision: encode_binary(revision)
-        )
-        response = GitalyClient.call(@repository.storage, :diff_service, :commit_patch, request, timeout: GitalyClient.medium_timeout)
-
-        response.sum(&:data)
-      end
-      # rubocop: enable CodeReuse/ActiveRecord
-
       def commit_stats(revision)
         request = Gitaly::CommitStatsRequest.new(
           repository: @gitaly_repo,

spec/lib/gitlab/gitaly_client/commit_service_spec.rb

@@ -252,31 +252,6 @@ describe Gitlab::GitalyClient::CommitService do
     end
   end
 
-  describe '#patch' do
-    let(:request) do
-      Gitaly::CommitPatchRequest.new(
-        repository: repository_message, revision: revision
-      )
-    end
-    let(:response) { [double(data: "my "), double(data: "diff")] }
-
-    subject { described_class.new(repository).patch(revision) }
-
-    it 'sends an RPC request' do
-      expect_any_instance_of(Gitaly::DiffService::Stub).to receive(:commit_patch)
-        .with(request, kind_of(Hash)).and_return([])
-
-      subject
-    end
-
-    it 'concatenates the responses data' do
-      allow_any_instance_of(Gitaly::DiffService::Stub).to receive(:commit_patch)
-        .with(request, kind_of(Hash)).and_return(response)
-
-      expect(subject).to eq("my diff")
-    end
-  end
-
   describe '#commit_stats' do
     let(:request) do
       Gitaly::CommitStatsRequest.new(