Bug 1592402 - Remove erroneous duplicate toolchain alias. r=froydnj

Differential Revision: https://phabricator.services.mozilla.com/D51003

--HG--
extra : moz-landing-system : lando

taskcluster/ci/toolchain/clang.yml

@@ -77,7 +77,6 @@ linux64-clang-9-cross:
             - 'build/build-clang/clang-linux64-cross.json'
         resources:
             - 'build/build-clang/clang-linux64-cross.json'
-        toolchain-alias: linux64-clang
         toolchain-artifact: public/build/clang.tar.xz
     fetches:
         fetch: