Cleanup unused variable with ext-tx experiment.
Cleanup unused variable with ext-tx experiment.

BUG=aomedia:446

Change-Id: I85433af2295c9610184ab5df812eabd8522689f6

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom