[LTO][Legacy] Eliminate an unused member of llvm::LTOModule. NFC.

[ikudrin]

Summary

This is a follow-up for D62935 where the corresponding API methods were removed.

[dblaikie]

@dexonsmith / @steven_wu - just a quick note that phab approvals without any text don't generate email/show up on the mailing list (leading to it looking like patches are committed without review approval) - if you can include some text in phab approvals it'd be helpful
maybe we'll be able to get this fixed in Phab at some point

[dexonsmith]

In D89472#2357507, @dblaikie wrote:

@dexonsmith / @steven_wu - just a quick note that phab approvals without any text don't generate email/show up on the mailing list (leading to it looking like patches are committed without review approval) - if you can include some text in phab approvals it'd be helpful
maybe we'll be able to get this fixed in Phab at some point

Thanks, noted! (I think I knew that before but forgot along the way...)

llvm/include/llvm/LTO/legacy/LTOModule.h

struct NameAndAttributes {
     bool               isFunction = 0;
     const GlobalValue *symbol = 0;
   };
 
   std::unique_ptr<LLVMContext> OwnedContext;
 
   std::string LinkerOpts;
 
-  std::string DependentLibraries;
-
   std::unique_ptr<Module> Mod;
   MemoryBufferRef MBRef;
   ModuleSymbolTable SymTab;
   std::unique_ptr<TargetMachine> _target;
   std::vector<NameAndAttributes> _symbols;
 
   // _defines and _undefines only needed to disambiguate tentative definitions
   StringSet<>                             _defines;