[AsmParser] Remove MDRef (NFC)

The last use was removed on Jan 12, 2015 in commit ab617d5.

Author: kazutakahirata

llvm/include/llvm/AsmParser/LLParser.h

@@ -93,21 +93,6 @@ namespace llvm {
  ModuleSummaryIndex *Index;
  SlotMapping *Slots;
 
- // Instruction metadata resolution. Each instruction can have a list of
- // MDRef info associated with them.
- //
- // The simpler approach of just creating temporary MDNodes and then calling
- // RAUW on them when the definition is processed doesn't work because some
- // instruction metadata kinds, such as dbg, get stored in the IR in an
- // "optimized" format which doesn't participate in the normal value use
- // lists. This means that RAUW doesn't work, even on temporary MDNodes
- // which otherwise support RAUW. Instead, we defer resolving MDNode
- // references until the definitions have been processed.
- struct MDRef {
- SMLoc Loc;
- unsigned MDKind, MDSlot;
- };
-
  SmallVector<Instruction*, 64> InstsWithTBAATag;
 
  // Type resolution handling data structures. The location is set when we