Update collect_all_artifacts.sh to remove C# Google.Protobuf.Tools nuget build (#13554)

CI shows this part of legacy release infrastructure is no longer working as expected due to failing keyserver request.

C# has already exited support window for 21.x so we are removing this artifact from releases on this line.

kokoro/release/collect_all_artifacts.sh

@@ -24,40 +24,3 @@ ls -R ${INPUT_ARTIFACTS_DIR}
 # TODO(jtattermusch): the directory layout of the artifact builds is pretty messy,
 # so will be the output artifacts of this job.
 cp -r ${INPUT_ARTIFACTS_DIR}/* artifacts
-
-# ====================================
-# Build Google.Protobuf.Tools C# nuget
-# The reason it's being done in this script is that we need access to protoc binaries
-# built on multiple platform (the build is performed by the "build artifact" step)
-# and adding and extra chained build just for building the Google.Protobuf.Tools
-# nuget seems like an overkill.
-cd csharp
-mkdir -p protoc/windows_x86
-mkdir -p protoc/windows_x64
-cp ${INPUT_ARTIFACTS_DIR}/build32/Release/protoc.exe protoc/windows_x86/protoc.exe
-cp ${INPUT_ARTIFACTS_DIR}/build64/Release/protoc.exe protoc/windows_x64/protoc.exe
-
-mkdir -p protoc/linux_x86
-mkdir -p protoc/linux_x64
-# Because of maven unrelated reasonse the linux protoc binaries have a dummy .exe extension.
-# For the Google.Protobuf.Tools nuget, we don't want that exception, so we just remove it.
-cp ${INPUT_ARTIFACTS_DIR}/protoc-artifacts/target/linux/x86_32/protoc.exe protoc/linux_x86/protoc
-cp ${INPUT_ARTIFACTS_DIR}/protoc-artifacts/target/linux/x86_64/protoc.exe protoc/linux_x64/protoc
-
-mkdir -p protoc/macosx_x64
-cp ${INPUT_ARTIFACTS_DIR}/build64/src/protoc protoc/macosx_x64/protoc
-
-# Install nuget (will also install  mono)
-# TODO(jtattermusch): use "mono:5.14" docker image instead so we don't have to apt-get install
-sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
-echo "deb https://download.mono-project.com/repo/ubuntu stable-xenial main" | sudo tee /etc/apt/sources.list.d/mono-official-stable.list
-# NVidia has stopped publishing Cuda packages for Ubuntu 16.04, so we need to
-# delete this file to allow the apt update to run successfully.
-sudo rm -f /etc/apt/sources.list.d/cuda.list
-sudo apt update
-sudo apt-get install -y nuget
-
-nuget pack Google.Protobuf.Tools.nuspec
-
-# Copy the nupkg to the output artifacts
-cp Google.Protobuf.Tools.*.nupkg ../artifacts