chore(ci): removing linkage monitor from v4 branch (#1027)

Author: suztomo

.github/workflows/ci.yaml

@@ -46,19 +46,6 @@ jobs:
         java-version: ${{matrix.java}}
     - run: java -version
     - run: .kokoro/dependencies.sh
-  linkage-monitor:
-    runs-on: ubuntu-latest
-    steps:
-    - uses: actions/checkout@v2
-    - uses: actions/setup-java@v1
-      with:
-        java-version: 8
-    - run: java -version
-    - name: Install artifacts to local Maven repository
-      run: .kokoro/build.sh
-      shell: bash
-    - name: Validate any conflicts with regard to com.google.cloud:libraries-bom (latest release)
-      uses: GoogleCloudPlatform/cloud-opensource-java/linkage-monitor@v1-linkagemonitor
   lint:
     runs-on: ubuntu-latest
     steps:
@@ -80,4 +67,4 @@ jobs:
     - run: java -version
     - run: .kokoro/build.sh
       env:
-        JOB_TYPE: clirr
+        JOB_TYPE: clirr