[ScheduleDAG] Remove obsolete VReg2SUnitMap. NFC (llvm#96559)

The VReg2SUnitMap type is not used anywhere (it has probably been
obsolete since commit 97d0ffb back in 2015). This commit
simply removes the type definition.

Author: bjope

llvm/include/llvm/CodeGen/ScheduleDAGInstrs.h

@@ -18,7 +18,6 @@
 #include "llvm/ADT/PointerIntPair.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/SparseMultiSet.h"
-#include "llvm/ADT/SparseSet.h"
 #include "llvm/ADT/identity.h"
 #include "llvm/CodeGen/LiveRegUnits.h"
 #include "llvm/CodeGen/MachineBasicBlock.h"
@@ -91,12 +90,6 @@ namespace llvm {
   using RegUnit2SUnitsMap =
       SparseMultiSet<PhysRegSUOper, identity<unsigned>, uint16_t>;
 
-  /// Use SparseSet as a SparseMap by relying on the fact that it never
-  /// compares ValueT's, only unsigned keys. This allows the set to be cleared
-  /// between scheduling regions in constant time as long as ValueT does not
-  /// require a destructor.
-  using VReg2SUnitMap = SparseSet<VReg2SUnit, VirtReg2IndexFunctor>;
-
   /// Track local uses of virtual registers. These uses are gathered by the DAG
   /// builder and may be consulted by the scheduler to avoid iterating an entire
   /// vreg use list.