Remove duplicate --bits command line parameter (#1287)

Remove duplicate --bits command line parameter. It's producing a warning with sox.

test/torchaudio_unittest/common_utils/sox_utils.py

@@ -49,8 +49,6 @@ def gen_audio_file(
     ]
     if compression is not None:
         command += ['--compression', str(compression)]
-    if bit_depth is not None:
-        command += ['--bits', str(bit_depth)]
     if encoding is not None:
         command += ['--encoding', str(encoding)]
     command += [