Revert "no_known_hosts is false"

This reverts commit 206d310c5c.

.github/workflows/verify-on-ubuntu.yml

@@ -278,23 +278,3 @@ jobs:
         no_known_hosts: true
     - name: print created files
       run: ls -l ~/.ssh
-  without-no_known_hosts:
-    name: no known_hosts file
-    runs-on: ${{ matrix.os }}
-    strategy:
-      fail-fast: false
-      matrix:
-        os:
-        - ubuntu-16.04
-        - ubuntu-18.04
-        - ubuntu-20.04
-    steps:
-    - name: Checkout source codes
-      uses: actions/checkout@v2
-    - name: Install SSH key
-      uses: ./.
-      with:
-        key: ${{ secrets.SSH_KEY_PEM }}
-      continue-on-error: true
-    - name: print created files
-      run: ls -l ~/.ssh