Trim xxhash CLI LICENSE (#26929)

Only the library part is used.

third_party/README.md

@@ -128,12 +128,15 @@ Since upb is vendored in the gRPC repo, you cannot use submodule to update it. P
 
 ### Updating third_party/xxhash
 
+TODO(https://github.com/Cyan4973/xxHash/issues/548): revisit LICENSE
+instructions if upstream splits library and CLI.
+
 The upstream xxhash repository contains a bunch of files that we don't want, so
 we employ a rather manual update flow to pick up just the bits we care about:
 
 ```
 git remote add xxhash https://github.com/Cyan4973/xxHash.git
 git fetch xxhash
-for file in LICENSE xxhash.h; do git show xxhash/dev:$file > third_party/xxhash/$file; done
-git commit
+git show xxhash/dev:xxhash.h > third_party/xxhash/xxhash.h
+git show xxhash/dev:LICENSE | sed -nE '/^-+$/q;p' > third_party/xxhash/LICENSE
 ```

third_party/xxhash/LICENSE

@@ -25,24 +25,3 @@ ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-----------------------------------------------------
-
-xxhsum command line interface
-Copyright (c) 2013-2020 Yann Collet
-All rights reserved.
-
-GPL v2 License
-
-This program is free software; you can redistribute it and/or modify
-it under the terms of the GNU General Public License as published by
-the Free Software Foundation; either version 2 of the License, or
-(at your option) any later version.
-
-This program is distributed in the hope that it will be useful,
-but WITHOUT ANY WARRANTY; without even the implied warranty of
-MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-GNU General Public License for more details.
-
-You should have received a copy of the GNU General Public License along
-with this program; if not, write to the Free Software Foundation, Inc.,
-51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.