[NFC] Remove unused code (KhronosGroup#5554)

Author: sudonatalie

source/opt/def_use_manager.h

@@ -27,28 +27,6 @@ namespace spvtools {
 namespace opt {
 namespace analysis {
 
-// Class for representing a use of id. Note that:
-// * Result type id is a use.
-// * Ids referenced in OpSectionMerge & OpLoopMerge are considered as use.
-// * Ids referenced in OpPhi's in operands are considered as use.
-struct Use {
-  Instruction* inst;       // Instruction using the id.
-  uint32_t operand_index;  // logical operand index of the id use. This can be
-                           // the index of result type id.
-};
-
-inline bool operator==(const Use& lhs, const Use& rhs) {
-  return lhs.inst == rhs.inst && lhs.operand_index == rhs.operand_index;
-}
-
-inline bool operator!=(const Use& lhs, const Use& rhs) { return !(lhs == rhs); }
-
-inline bool operator<(const Use& lhs, const Use& rhs) {
-  if (lhs.inst < rhs.inst) return true;
-  if (lhs.inst > rhs.inst) return false;
-  return lhs.operand_index < rhs.operand_index;
-}
-
 // Definition should never be null. User can be null, however, such an entry
 // should be used only for searching (e.g. all users of a particular definition)
 // and never stored in a container.