Drop windows test version due to LLVM issue

Author: mewilhel

.github/workflows/ci.yml

@@ -18,15 +18,9 @@ jobs:
           - version: '1.6'
             os: ubuntu-latest
             arch: x64
-          - version: '1.6'
-            os: windows-latest
-            arch: x64
           - version: '1.7'
             os: ubuntu-latest
             arch: x64
-          - version: '1.7'
-            os: windows-latest
-            arch: x64
     steps:
       - uses: actions/checkout@v2
       - uses: julia-actions/setup-julia@v1