[CI] The checkstyle workflow is redundant

[shink]

The checkstyle plugin is bound to Maven's validate phase. When execute mvn verify, code will be checked. So the checkstyle workflow is redundant.

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-checkstyle-plugin</artifactId>
  <version>3.1.1</version>
  <dependencies>
    <dependency>
      <groupId>com.puppycrawl.tools</groupId>
      <artifactId>checkstyle</artifactId>
      <version>8.29</version>
    </dependency>
  </dependencies>
  <executions>
    <execution>
      <id>checkstyle</id>
      <phase>validate</phase>
      <goals>
        <goal>check</goal>
      </goals>
    </execution>
  </executions>
</plugin>