[NFC][SPIRV] remove unimplemented function define for selectCross (#134478)

- `selectCross` looks to be a function that had its implementation and
usage removed but this define some how stuck around.
- this change removes the definition.

Author: farzonl

llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp

@@ -183,8 +183,6 @@ class SPIRVInstructionSelector : public InstructionSelector {
 
   bool selectCmp(Register ResVReg, const SPIRVType *ResType,
                  unsigned comparisonOpcode, MachineInstr &I) const;
-  bool selectCross(Register ResVReg, const SPIRVType *ResType,
-                   MachineInstr &I) const;
   bool selectDiscard(Register ResVReg, const SPIRVType *ResType,
                      MachineInstr &I) const;