Remove another unused function from ModuleManager. We have no notion of a 'last' module any more

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@138081 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/clang/Serialization/ASTReader.h b/include/clang/Serialization/ASTReader.h
index bd20601..5223bd8 100644
--- a/include/clang/Serialization/ASTReader.h
+++ b/include/clang/Serialization/ASTReader.h
@@ -482,10 +482,6 @@
   /// the first module loaded.
   Module &getPrimaryModule() const { return *Chain[0]; }
 
-  /// \brief Returns the latest module associated with the manager, that is,
-  /// the last module loaded
-  Module &getLastModule() { return *Chain.back(); }
-
   /// \brief Returns the module associated with the given index
   Module &operator[](unsigned Index) const { return *Chain[Index]; }