Remove Opus from checklicenses whitelist

The license header has been added to
third_party/opus/src/autogen.sh upstream and updated here

R=thestig@chromium.org

Change-Id: I8e1adf14995d6d184edf4e77a77e0991516df7d1
Reviewed-on: https://chromium-review.googlesource.com/1062068Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Felicia Lim <flim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561686}

tools/checklicenses/checklicenses.py

@@ -468,9 +468,6 @@ PATH_SPECIFIC_WHITELISTED_LICENSES = {
     'third_party/openmax_dl/dl' : [
         'Khronos Group',
     ],
-    'third_party/opus/src/autogen.sh' : [  # https://trac.xiph.org/ticket/2253#ticket
-        'UNKNOWN',
-    ],
     'third_party/boringssl': [
         # There are some files in BoringSSL which came from OpenSSL and have no
         # license in them. We don't wish to add the license header ourselves