SPARK-4305 [BUILD] yarn-alpha profile won't build due to network/yarn module

SPARK-3797 introduced the `network/yarn` module, but its YARN code depends on YARN APIs not present in older versions covered by the `yarn-alpha` profile. As a result builds like `mvn -Pyarn-alpha -Phadoop-0.23 -Dhadoop.version=0.23.7 -DskipTests clean package` fail.

The solution is just to not build `network/yarn` with profile `yarn-alpha`.

Author: Sean Owen <sowen@cloudera.com>

Closes #3167 from srowen/SPARK-4305 and squashes the following commits:

88938cb [Sean Owen] Don't build network/yarn in yarn-alpha profile as it won't compile

pom.xml

@@ -1229,7 +1229,6 @@
       <id>yarn-alpha</id>
       <modules>
         <module>yarn</module>
-        <module>network/yarn</module>
       </modules>
     </profile>