Remove ExtractCommitSignature RPC

The RPC has gone unused as it didn't scale, and was replaced with
GetCommitSignatures. There's no active code path that calls this RPC and
Gitaly metrics confirms that it's gone unused.

Part of: https://gitlab.com/gitlab-org/gitlab/issues/31079

lib/gitlab/git/commit.rb

@@ -155,10 +155,6 @@ module Gitlab
           end
         end
 
-        def extract_signature(repository, commit_id)
-          repository.gitaly_commit_client.extract_signature(commit_id)
-        end
-
         def extract_signature_lazily(repository, commit_id)
           BatchLoader.for(commit_id).batch(key: repository) do |commit_ids, loader, args|
             batch_signature_extraction(args[:key], commit_ids).each do |commit_id, signature_data|

lib/gitlab/gitaly_client/commit_service.rb

@@ -348,25 +348,6 @@ module Gitlab
         end
       end
 
-      def extract_signature(commit_id)
-        request = Gitaly::ExtractCommitSignatureRequest.new(repository: @gitaly_repo, commit_id: commit_id)
-        response = GitalyClient.call(@repository.storage, :commit_service, :extract_commit_signature, request, timeout: GitalyClient.fast_timeout)
-
-        signature = +''.b
-        signed_text = +''.b
-
-        response.each do |message|
-          signature << message.signature
-          signed_text << message.signed_text
-        end
-
-        return if signature.blank? && signed_text.blank?
-
-        [signature, signed_text]
-      rescue GRPC::InvalidArgument => ex
-        raise ArgumentError, ex
-      end
-
       def get_commit_signatures(commit_ids)
         request = Gitaly::GetCommitSignaturesRequest.new(repository: @gitaly_repo, commit_ids: commit_ids)
         response = GitalyClient.call(@repository.storage, :commit_service, :get_commit_signatures, request, timeout: GitalyClient.fast_timeout)

spec/lib/gitlab/git/commit_spec.rb

@@ -521,12 +521,6 @@ describe Gitlab::Git::Commit, :seed_helper do
 
       it_behaves_like 'extracting commit signature'
     end
-
-    describe '.extract_signature' do
-      subject { described_class.extract_signature(repository, commit_id) }
-
-      it_behaves_like 'extracting commit signature'
-    end
   end
 
   skip 'move this test to gitaly-ruby' do