fix Grpc.csproj build, add Grpc.csproj to the solution

src/csharp/Grpc.sln

@@ -49,6 +49,8 @@ Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Grpc.IntegrationTesting.Xds
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Grpc.Core.Xamarin", "Grpc.Core.Xamarin\Grpc.Core.Xamarin.csproj", "{1DCB698B-1383-4297-975A-EC8383295141}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Grpc", "Grpc\Grpc.csproj", "{6029123D-BA2B-4333-A2FD-67FB1F6CBBCD}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -147,6 +149,10 @@ Global
 		{1DCB698B-1383-4297-975A-EC8383295141}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{1DCB698B-1383-4297-975A-EC8383295141}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{1DCB698B-1383-4297-975A-EC8383295141}.Release|Any CPU.Build.0 = Release|Any CPU
+		{6029123D-BA2B-4333-A2FD-67FB1F6CBBCD}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{6029123D-BA2B-4333-A2FD-67FB1F6CBBCD}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{6029123D-BA2B-4333-A2FD-67FB1F6CBBCD}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{6029123D-BA2B-4333-A2FD-67FB1F6CBBCD}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE

src/csharp/Grpc/Grpc.csproj

@@ -1,7 +1,4 @@
 <Project Sdk="Microsoft.NET.Sdk">
-
-  <Import Project="..\Grpc.Core\Common.csproj.include" />
-
   <PropertyGroup>
     <Authors>The gRPC Authors</Authors>
     <Copyright>Copyright 2015 The gRPC Authors</Copyright>