[lldb] Remove unused private field 'm_orig_rax_info' in RegisterContextLinux_x86_64.h (NFC)

/data/llvm-project/lldb/source/Plugins/Process/Utility/RegisterContextLinux_x86_64.h:32:30: error: private field 'm_orig_rax_info' is not used [-Werror,-Wunused-private-field]
  lldb_private::RegisterInfo m_orig_rax_info;
                             ^
1 error generated.