Cleanup netfx-specific code from projects that no longer build in netfx configuration...

Cleanup netfx-specific code from projects that no longer build in netfx configurations (dotnet/corefx#31818)

* Remove Crc32Helper.Managed.cs and '$(TargetGroup)' == 'netfx' conditionals from System.IO.Compression.csproj

* Remove netfx conditionals from System.Data.Common.csproj

* Remove netfx conditionals from System.Diagnostics.StackTrace.csproj

* Remove 'netfx' and 'net46' conditionals from System.Net.Http.csproj

* Remove 'netfx' conditionals from System.Net.Sockets.csproj

* Remove 'netfx', 'net462' and 'net47' conditionals from System.Runtime.InteropServices.RuntimeInformation.csproj

* Remove 'netfx' conditionals from System.Runtime.Serialization.Primitives.csproj

* Remove 'netfx' conditionals from System.Runtime.Serialization.Xml.csproj

* Remove 'netfx' and 'net47' conditionals from System.Security.Cryptography.Algorithms.csproj

* Remove 'netfx' conditionals from System.Security.SecureString.csproj

* Remove 'netfx' conditionals from System.Threading.Overlapped.csproj

* Remove 'netfx' conditionals from System.Xml.XPath.XDocument.csproj

* Trim out unused source file DeferredDisposableLifetime.cs

Commit migrated from https://github.com/dotnet/corefx/commit/6a0ecec8279504c03433f689e4a7cf6cd63da27c