fix: Remove useless combine.children=append

Since forkMode is a single element option, we don't need `combine.children` option.

- ref: https://maven.apache.org/pom.html

```
[ERROR] Failed to execute goal org.codehaus.mojo:xml-maven-plugin:1.0.1:validate (default-cli) on project zeppelin-zengine: While parsing /Users/lambda/github/apache-zeppelin/zeppelin-master/zeppelin-zengine/pom.xml, at file:/Users/lambda/github/apache-zeppelin/zeppelin-master/zeppelin-zengine/pom.xml, line 253,  column 50: cvc-complex-type.3.2.2: Attribute 'combine.children' is not allowed to appearin element 'configuration'. -> [Help 1]
```

maven effective pom shows same result after removing that option

```
// after removed
<plugin>
        <artifactId>maven-surefire-plugin</artifactId>
        <version>2.17</version>
        <executions>
          <execution>
            <id>default-test</id>
            <phase>test</phase>
            <goals>
              <goal>test</goal>
            </goals>
            <configuration>
              <forkMode>always</forkMode>
              <argLine>-Xmx2g -Xms1g -Dfile.encoding=UTF-8</argLine>
            </configuration>
          </execution>
        </executions>
```

zeppelin-zengine/pom.xml

@@ -250,7 +250,7 @@
       <plugin>
         <artifactId>maven-surefire-plugin</artifactId>
         <version>2.17</version>
-        <configuration combine.children="append">
+        <configuration>
           <forkMode>always</forkMode>
         </configuration>
       </plugin>