remove CI for unsupported os (#396)

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index d207d8c..0c2d2ea 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -28,7 +28,7 @@
     strategy:
       matrix:
         node-version: [10.x, 12.x, 14.x, 16.x]
-        os: [ubuntu-latest, windows-latest, macos-latest]
+        os: [windows-latest]
 
     steps:
       - uses: actions/checkout@v2
@@ -38,16 +38,10 @@
         with:
           node-version: ${{ matrix.node-version }}
 
-      - name: set up JDK 1.8
-        uses: actions/setup-java@v1
-        with:
-          java-version: 1.8
-
       - name: Environment Information
         run: |
           node --version
           npm --version
-          gradle --version
 
       - name: npm install and test
         run: |