[SystemZ] Remove unused function 'decodeU6ImmOperand' in SystemZDisassembler.cpp (NFC)

/data/llvm-project/llvm/lib/Target/SystemZ/Disassembler/SystemZDisassembler.cpp:209:21: error: unused function 'decodeU6ImmOperand' [-Werror,-Wunused-function]
static DecodeStatus decodeU6ImmOperand(MCInst &Inst, uint64_t Imm,
                    ^
1 error generated.