README incorrectly suggests build sources spark-env.sh

This is misleading because the build doesn't source that file. IMO
it's better to force people to specify build environment variables
on the command line always, like we do in every example.

README.md

@@ -69,9 +69,6 @@ When building for Hadoop 2.2.X and newer, you'll need to include the additional
     # Apache Hadoop 2.2.X and newer
     $ mvn -Dyarn.version=2.2.0 -Dhadoop.version=2.2.0 -Pnew-yarn
 
-For convenience, these variables may also be set through the `conf/spark-env.sh` file
-described below.
-
 When developing a Spark application, specify the Hadoop version by adding the
 "hadoop-client" artifact to your project's dependencies. For example, if you're
 using Hadoop 1.2.1 and build your application using SBT, add this entry to