[Support] Remove an outdated comment.

[ioeric]

Summary

The comment about removeArgument seems to be outdated after
https://reviews.llvm.org/rL227345

I came across a use case where options only live in a function scope and need to
be unregistered before destruction, and It seems that removeArgument works
fine now after https://reviews.llvm.org/rL227345

[beanz]

LGTM. I am curious what your use case for adding and removing options is, can you elaborate?

include/llvm/Support/CommandLine.h

 
 public:
   virtual ~Option() = default;
 
   // addArgument - Register this argument with the commandline system.
   //
   void addArgument();
 
-  /// Unregisters this option from the CommandLine system.
-  ///
-  /// This option must have been the last option registered.
-  /// For testing purposes only.
+  // Unregisters this option from the CommandLine system.
   void removeArgument();
 
   // Return the width of the option tag for printing...
   virtual size_t getOptionWidth() const = 0;
 
   // printOptionInfo - Print out information about this option.  The
   // to-be-maintained width is specified.
   //