[AST] NFC: Remove unused field

This doesn't appear to have ever been used.

Author: hamishknight

lib/AST/ASTContext.cpp

@@ -162,9 +162,6 @@ struct ASTContext::Implementation {
   /// DenseMap.
   llvm::MapVector<Identifier, ModuleDecl *> LoadedModules;
 
-  /// The set of top-level modules we have loaded, indexed by ABI name.
-  llvm::MapVector<Identifier, ModuleDecl *> LoadedModulesByABIName;
-
   // FIXME: This is a StringMap rather than a StringSet because StringSet
   // doesn't allow passing in a pre-existing allocator.
   llvm::StringMap<Identifier::Aligner, llvm::BumpPtrAllocator&>