cmMakefile: Mark definitions explicitly erased, even at top level.

Presumably the intention here is to attempt to optimize memory by not
storing what is not needed.  However, all keys need to be tracked
anyway to implement initialization tracking, and this special case
gets in the way of simplifying the implementation of that.

This doesn't change any observable effects because values set
to 0 are considered not to exist by the cmDefinitions API.

Source/cmDefinitions.cxx

@@ -61,11 +61,6 @@ void cmDefinitions::Set(const std::string& key, const char* value)
   this->Map[key] = def;
 }
 
-void cmDefinitions::Erase(const std::string& key)
-{
-  this->Map.erase(key);
-}
-
 //----------------------------------------------------------------------------
 std::vector<std::string> cmDefinitions::LocalKeys() const
 {

Source/cmDefinitions.h

@@ -43,8 +43,6 @@ public:
   /** Set (or unset if null) a value associated with a key.  */
   void Set(const std::string& key, const char* value);
 
-  void Erase(const std::string& key);
-
   /** Get the set of all local keys.  */
   std::vector<std::string> LocalKeys() const;
 

Source/cmMakefile.cxx

@@ -76,16 +76,8 @@ public:
 
   void RemoveDefinition(std::string const& name)
   {
-    if (this->VarStack.size() > 1)
-      {
-      // In lower scopes we store keys, defined or not.
     this->VarStack.back().Set(name, 0);
   }
-    else
-      {
-      this->VarStack.back().Erase(name);
-      }
-  }
 
   std::vector<std::string> LocalKeys() const
   {