ENH: Remove SKIP_RULE_DEPENDS option from add_custom_command()

- Option was recently added but never released.
  - Custom commands no longer depend on build.make so we do
    not need the option.
  - Rule hashes now take care of rebuilding when rules change
    so the dependency is not needed.

Source/cmCustomCommand.h

@@ -68,10 +68,6 @@ public:
   bool GetEscapeAllowMakeVars() const;
   void SetEscapeAllowMakeVars(bool b);
 
-  /** Set/Get whether to skip the dependency on the rule itself.  */
-  bool GetSkipRuleDepends() const;
-  void SetSkipRuleDepends(bool b);
-
   typedef std::pair<cmStdString, cmStdString> ImplicitDependsPair;
   class ImplicitDependsList: public std::vector<ImplicitDependsPair> {};
   void SetImplicitDepends(ImplicitDependsList const&);
@@ -87,7 +83,6 @@ private:
   std::string WorkingDirectory;
   bool EscapeAllowMakeVars;
   bool EscapeOldStyle;
-  bool SkipRuleDepends;
   ImplicitDependsList ImplicitDepends;
 };