(tsoding#8) Remove CXX env from CI

.github/workflows/ci.yml

@@ -15,7 +15,6 @@ jobs:
           make
         env:
           CC: gcc
-          CXX: g++
   build-linux-clang:
     runs-on: ubuntu-18.04
     steps:
@@ -29,7 +28,6 @@ jobs:
           make
         env:
           CC: clang
-          CXX: clang++
   build-macos:
     runs-on: macOS-latest
     steps:
@@ -41,5 +39,4 @@ jobs:
           make
         env:
           CC: clang
-          CXX: clang++
   # TODO: there is no build for Windows