cmCommand: Remove IsDiscouraged interface.

This used to affect documentation, but does not affect the Sphinx
documentation.

Source/cmSubdirDependsCommand.h

@@ -21,7 +21,6 @@ public:
   virtual bool InitialPass(std::vector<std::string> const& args,
                            cmExecutionStatus &status);
   virtual std::string GetName() const { return "subdir_depends";}
-  virtual bool IsDiscouraged() const { return true; }
   cmTypeMacro(cmSubdirDependsCommand, cmCommand);
 };