Remove more dead code

[bartdesmet]

Remove some code that's no longer used, and simplify the exception singleton used. It doesn't need to have a unique type (and we never perform type checks); we should just never leak the instance. Also addresses a few warnings:

src\System.Reactive\Internal\ExceptionHelper.cs(101,30): warning CA1032: Add the following constructor to TerminatedException: public TerminatedException(string message)
src\System.Reactive\Internal\ExceptionHelper.cs(101,30): warning CA1032: Add the following constructor to TerminatedException: public TerminatedException(string message, Exception innerException)
src\System.Reactive\Internal\ExceptionHelper.cs(101,30): warning CA1064: Exceptions should be public