Remove unnecessary System.Linq usage from Microsoft.CSharp (dotnet/corefx#41552)

Remove unnecessary System.Linq usage from Microsoft.CSharp (dotnet/corefx#41552)

Most of the usage was just as easily (if not more easily) done without LINQ.  In some cases the use was entirely superfluous (e.g. MethodReturningNewArray().ToArray()).

Commit migrated from https://github.com/dotnet/corefx/commit/09698dd91c53e123d3a0f1240b5f611b3c14639c