Don't link to removed thread library

[jbuck]

Hi there, I've been working on getting this working for a project at work. In kaldi-asr/kaldi@6cc8e3a the kaldi project removed the vendored pthread library to use the STL thread library. This patch removes the now-deleted lib so it can compile with the latest kaldi master branch.

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[realill]

@jbuck please sign CLA before I can merge this.

[jbuck]

I signed it!

[googlebot]

CLAs look good, thanks!

[achernetsov]

Hello, @realill, are you planning to merge this?