fix: remove git submodule from project (#586)

Remove the permanent git submodule from the project, and only add it on-demand
when re-generating code from the proto definition for the gRPC API.

Author: olavloite

.gitmodules

@@ -1,4 +1,5 @@
 PATH="${PATH}:${HOME}/go/bin"
+git submodule add git@github.com:googleapis/googleapis.git
 ln -sf "${PWD}"/google/spannerlib googleapis/google/spannerlib
 cd googleapis || exit 1
 protoc \
@@ -13,13 +14,17 @@ protoc \
   --java-grpc_out=../../wrappers/spannerlib-java/src/main/java/ \
   --java-grpc_opt=paths=source_relative \
   google/spannerlib/v1/spannerlib.proto
+
+# dotnet add package Grpc.Tools --version 2.76.0
 protoc \
   --csharp_out=../../wrappers/spannerlib-dotnet/spannerlib-dotnet-grpc-v1/ \
-  --plugin=protoc-gen-csharp_grpc=/Users/loite/.nuget/packages/grpc.tools/2.72.0/tools/macosx_x64/grpc_csharp_plugin \
+  --plugin=protoc-gen-csharp_grpc=/Users/loite/.nuget/packages/grpc.tools/2.76.0/tools/macosx_x64/grpc_csharp_plugin \
   --csharp_grpc_out=../../wrappers/spannerlib-dotnet/spannerlib-dotnet-grpc-v1/ \
   --csharp_opt=file_extension=.g.cs \
   --csharp_grpc_opt=no_server \
   --proto_path=. \
   google/spannerlib/v1/spannerlib.proto
 cd .. || exit 1
 rm googleapis/google/spannerlib
+git rm googleapis
+rm ../../.gitmodules