[MINOR][BUILD] Remove useless add-source

## What changes were proposed in this pull request?

The source directory(`sql/hive-thriftserver/v${hive.version.short}/src/main/scala`) removed from  [SPARK-6909](https://issues.apache.org/jira/browse/SPARK-6909) and [SPARK-7850](https://issues.apache.org/jira/browse/SPARK-7850). We should also remove the `add-source`.
It seems that removed this `add-source` makes it easier to import `src/gen` source code in IDEA:
![image](https://user-images.githubusercontent.com/5399861/53715396-a967b380-3e8c-11e9-8aa1-c59d819b4c06.png)
![image](https://user-images.githubusercontent.com/5399861/53715402-acfb3a80-3e8c-11e9-8aa9-a716931160c6.png)

## How was this patch tested?

manual tests

Closes #23949 from wangyum/SPARK-7850.

Authored-by: Yuming Wang <[email protected]>
Signed-off-by: Sean Owen <[email protected]>

Author: wangyum

sql/hive-thriftserver/pom.xml

@@ -120,18 +120,6 @@
         <groupId>org.codehaus.mojo</groupId>
         <artifactId>build-helper-maven-plugin</artifactId>
         <executions>
-          <execution>
-            <id>add-default-sources</id>
-            <phase>generate-sources</phase>
-            <goals>
-              <goal>add-source</goal>
-            </goals>
-            <configuration>
-              <sources>
-                <source>v${hive.version.short}/src/main/scala</source>
-              </sources>
-            </configuration>
-          </execution>
           <execution>
             <id>add-source</id>
             <phase>generate-sources</phase>