C#: Remove version constraints on most dependencies.

This means that `dotnet paket update` will pull in the
latest matching versions of all dependencies, except
the pinned roslyn versions.
GitInfo also remains pinned, as we're removing that
dependency when converting the build to bazel anyways.

This removes a whole bunch of transitive dependencies
we no longer need, resulting in a smaller distribution,
and hopefully a smaller build.

Author: criemen

csharp/paket.dependencies

@@ -4,17 +4,17 @@ source https://api.nuget.org/v3/index.json
 # behave like nuget in choosing transitive dependency versions
 strategy: min
 
-nuget Mono.Posix.NETStandard 1.0.0
-nuget Newtonsoft.Json 13.0.3
-nuget xunit 2.6.2
-nuget xunit.runner.visualstudio 2.5.4
-nuget Microsoft.NET.Test.Sdk 17.8.0
+nuget Mono.Posix.NETStandard
+nuget Newtonsoft.Json
+nuget xunit
+nuget xunit.runner.visualstudio
+nuget Microsoft.NET.Test.Sdk
 nuget Microsoft.CodeAnalysis.CSharp 4.8.0
 nuget Microsoft.CodeAnalysis 4.8.0
 nuget Microsoft.Build 17.8.3
-nuget Microsoft.Win32.Primitives 4.3.0
-nuget System.Net.Primitives 4.3.1
-nuget System.Security.Principal 4.3.0
-nuget System.Threading.ThreadPool 4.3.0
-nuget System.IO.FileSystem 4.3.0
+nuget Microsoft.Win32.Primitives
+nuget System.Net.Primitives
+nuget System.Security.Principal
+nuget System.Threading.ThreadPool
+nuget System.IO.FileSystem
 nuget GitInfo 3.3.3