cmTarget: Drop 'head' argument from GetLanguages

No call sites need it to be anything but 'this'.

Source/cmTarget.cxx

@@ -5419,11 +5419,10 @@ cmTarget::GetObjectLibrariesCMP0026(std::vector<cmTarget*>& objlibs) const
 
 //----------------------------------------------------------------------------
 void cmTarget::GetLanguages(std::set<std::string>& languages,
-                            const std::string& config,
-                            cmTarget const* head) const
+                            const std::string& config) const
 {
   std::vector<cmSourceFile*> sourceFiles;
-  this->GetSourceFiles(sourceFiles, config, head);
+  this->GetSourceFiles(sourceFiles, config, this);
   for(std::vector<cmSourceFile*>::const_iterator
         i = sourceFiles.begin(); i != sourceFiles.end(); ++i)
     {
@@ -5459,7 +5458,7 @@ void cmTarget::GetLanguages(std::set<std::string>& languages,
   for(std::vector<cmTarget*>::const_iterator
       i = objectLibraries.begin(); i != objectLibraries.end(); ++i)
     {
-    (*i)->GetLanguages(languages, config, head);
+    (*i)->GetLanguages(languages, config);
     }
 }
 
@@ -6551,7 +6550,7 @@ cmTarget::ComputeLinkImplementationLanguages(const std::string& config,
   // This target needs runtime libraries for its source languages.
   std::set<std::string> languages;
   // Get languages used in our source files.
-  this->GetLanguages(languages, config, this);
+  this->GetLanguages(languages, config);
   // Copy the set of langauges to the link implementation.
   for(std::set<std::string>::iterator li = languages.begin();
       li != languages.end(); ++li)

Source/cmTarget.h

@@ -491,8 +491,7 @@ public:
   // information to forward these property changes to the targets
   // until we have per-target object file properties.
   void GetLanguages(std::set<std::string>& languages,
-                    std::string const& config,
-                    cmTarget const* head = 0) const;
+                    std::string const& config) const;
 
   /** Return whether this target is an executable with symbol exports
       enabled.  */