remove LICENSE_THIRD_PARTY

The file this referes to (x86inc.asm) was removed in commit
c9986313

BUG=libyuv:790

Change-Id: I4aae28475ef11e01304daf2aaf60a0404752cfa7
Reviewed-on: https://chromium-review.googlesource.com/996373Reviewed-by: Frank Barchard <fbarchard@chromium.org>
Commit-Queue: Frank Barchard <fbarchard@chromium.org>

LICENSE_THIRD_PARTY

-This source tree contains third party source code which is governed by third
-party licenses. This file contains references to files which are under other
-licenses than the one provided in the LICENSE file in the root of the source
-tree.
-
-Files governed by third party licenses:
-source/x86inc.asm
-