[SPARK-4953][Doc] Fix the description of building Spark with YARN

At the section "Specifying the Hadoop Version" In building-spark.md, there is description about building with YARN with Hadoop 0.23.
Spark 1.3.0 will not support Hadoop 0.23 so we should fix the description.
This is not only applicable to CDH *5*+, so I'd revert that addition. What was removed with `yarn-alpha` was not really "Hadoop 0.23" support, although it kind of lines up with that. Why not remove this whole qualifying "For Apache Hadoop ..." phrase altogether? Also, do you mean Spark 1.2? what are you referring to in 1.3 otherwise?