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/cmMakefileExecutableTargetGenerator.cxx

@@ -69,9 +69,6 @@ void cmMakefileExecutableTargetGenerator::WriteRuleFiles()
     this->WriteExecutableRule(true);
   }
 
-  // Write the requires target.
-  this->WriteTargetRequiresRules();
-
   // Write clean target
   this->WriteTargetCleanRules();