[CodeGen] Remove unused lambda capture (NFC)

llvm-project/llvm/lib/CodeGen/MachineLoopInfo.cpp:215:14:
error: lambda capture 'Reg' is not used [-Werror,-Wunused-lambda-capture]
      [this, Reg](const MachineInstr &MI) { return this->contains(&MI); });
           ~~^~~
1 error generated.