Replace Kaldi mutex with STL mutex

[entn-at]

Recent versions of Kaldi removed the threading libraries (thread/kaldi-mutex.h etc.) and rely on STL implementations (std::mutex).

[mrsrikanth]

Hello,
Thanks for the pull request, but this is isn't really necessary anymore.
I already fixed this in issue #4. I hadn't realized that not all changes were pushed until
now.

Thanks for your effort.

• Srikanth