SILInstructions: remove two unused fields.

Fixes a compiler warning.
NFC

Author: eeckstein

include/swift/SIL/SILInstruction.h

@@ -6624,8 +6624,6 @@ class BeginCOWMutationInst final
   friend SILBuilder;
   friend TrailingObjects;
 
-  bool native;
-
   BeginCOWMutationInst(SILDebugLocation loc, SILValue operand,
                        ArrayRef<SILType> resultTypes,
                        ArrayRef<ValueOwnershipKind> resultOwnerships,
@@ -6670,8 +6668,6 @@ class EndCOWMutationInst
 {
   friend SILBuilder;
 
-  bool keepUnique;
-
   EndCOWMutationInst(SILDebugLocation DebugLoc, SILValue Operand,
                      bool keepUnique)
       : UnaryInstructionBase(DebugLoc, Operand, Operand->getType()) {