Bug 1478919 - Switch Spidermonkey TSAN builds to the default clang. r=dmajor

It was kept on clang 5 explicitly in bug 1467658 because of bug
1467673, now fixed.

--HG--
extra : rebase_source : 8de52e6967bb1f249b7e59d83b90ecfb291a9c44

taskcluster/ci/spidermonkey/linux.yml

@@ -139,8 +139,6 @@ sm-tsan-linux64/opt:
         platform: linux64/opt
     run:
         spidermonkey-variant: tsan
-    toolchains:
-        - linux64-clang-5
 
 sm-rootanalysis-linux64/debug:
     description: "Spidermonkey Root Analysis"