[Hexagon] Remove unused declaration CanReturnSmallStruct (NFC)

The declaration was introduced without a corresponding definition on
Dec 12, 2011 in commit 1213a7a57f.

llvm/lib/Target/Hexagon/HexagonISelLowering.h

@@ -107,9 +107,6 @@ class HexagonTargetLowering : public TargetLowering {
   const HexagonTargetMachine &HTM;
   const HexagonSubtarget &Subtarget;
 
-  bool CanReturnSmallStruct(const Function* CalleeFn, unsigned& RetSize)
-      const;
-
 public:
   explicit HexagonTargetLowering(const TargetMachine &TM,
                                  const HexagonSubtarget &ST);