Remove cxxstd=1z from clang-5.0

Author: pdimov

.github/workflows/ci.yml

@@ -83,7 +83,7 @@ jobs:
             install: clang-4.0
           - toolset: clang
             compiler: clang++-5.0
-            cxxstd: "03,11,14,1z"
+            cxxstd: "03,11,14"
             os: ubuntu-latest
             container: ubuntu:18.04
             install: clang-5.0