Revert labeler action and remove .travis.yml (#3504)

* Revert labeler

* Remove .travis.yml as well

Remove .travis.yml as well since it's disabled permanently

.github/labeler.yml

-document:
-  - "**/*.md"
-
-agent:
-  - "apm-sniffer/apm-sdk-plugin/**"
-  - "apm-sniffer/optional-plugins/**"
-
-plugin:
-  - "apm-sniffer/apm-sdk-plugin/**"
-  - "apm-sniffer/optional-plugins/**"
-  - "oap-server/server-cluster-plugin/**"
-  - "oap-server/configuration/**"
-  - "oap-server/storage-plugin/**"
-  - "oap-server/telemetry/**"
-  - "oap-server/receiver-plugin/**"
-
-core feature:
-  - "apm-sniffer/apm-agent-core/**"
-  - "oap-server/server-core/**"
-  - "oap-server/server-library/**"
-  - "oap-server/oal-grammar/**"
-  - "oap-server/oal-rt/**"
-
-docker:
-  - "docker/**"
-
-OAP-backend:
-  - "oap-server/**"
-
-UI:
-  - "oap-server/server-query-plugin/**"
-  - "skywalking-ui/**"
-
-submodule update:
-  - ".gitmodules"
-
-CI settings:
-  - "appveyor.yml"
-  - "Jenkinsfile"
-  - "Jenkinsfile-E2E"
-  - ".github/**"
-  - "tools/**"
-
-test:
-  - "test/**"
\ No newline at end of file

.github/workflows/labeler.yml

-name: Labeler
-on: [pull_request]
-  
-jobs:
-  label:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v1
-        with:
-          submodules: recursive
-      - uses: actions/labeler@v2
-        with:
-          repo-token: "${{ secrets.GITHUB_TOKEN }}"

.travis.yml

-sudo: required
-
-os:
-  - linux
-
-jdk:
-  - openjdk8
-
-language: java
-
-install:
-  - ./mvnw -DskipTests clean install --quiet