Makefiles: Drop 'requires' step and its supporting infrastructure

The 'requires' step was used to provide implicit dependencies between
the generated Fortran module files and a Fortran target that needs these
module files to ensure the correct compilation order.  After recent
refactoring to resolve all dependencies explicitly through `.mod.stamp`
make targets, the separate 'requires' step is not needed anymore.

Source/cmGlobalUnixMakefileGenerator3.h

@@ -174,9 +174,6 @@ protected:
   void AppendGlobalTargetDepends(std::vector<std::string>& depends,
                                  cmGeneratorTarget* target);
 
-  // does this generator need a requires step for any of its targets
-  bool NeedRequiresStep(cmGeneratorTarget const*);
-
   // Target name hooks for superclass.
   const char* GetAllTargetName() const override { return "all"; }
   const char* GetInstallTargetName() const override { return "install"; }