Bug 1485219 - Switch valgrind builds to clang. r=chmanchester

taskcluster/ci/valgrind/kind.yml

@@ -31,7 +31,6 @@ jobs:
             max-run-time: 72000
             env:
                 PERFHERDER_EXTRA_OPTIONS: valgrind
-                FORCE_GCC: '1'
         run:
             using: mozharness
             actions: [get-secrets build valgrind-test]