[AMDGPU] Remove unused lambda capture (NFC)

/llvm-project/llvm/lib/Target/AMDGPU/AMDGPUAsmPrinter.cpp:733:30:
error: lambda capture 'Ctx' is not used [-Werror,-Wunused-lambda-capture]
  auto TryGetMCExprValue = [&Ctx](const MCExpr *Value, uint64_t &Res) -> bool {
                            ~^~~
1 error generated.