[SPARK-14744][EXAMPLES] Clean up examples packaging, remove outdated examples.

First, make all dependencies in the examples module provided, and explicitly
list a couple of ones that somehow are promoted to compile by maven. This
means that to run streaming examples, the streaming connector package needs
to be provided to run-examples using --packages or --jars, just like regular
apps.

Also, remove a couple of outdated examples. HBase has had Spark bindings for
a while and is even including them in the HBase distribution in the next
version, making the examples obsolete. The same applies to Cassandra, which
seems to have a proper Spark binding library already.

I just tested the build, which passes, and ran SparkPi. The examples jars
directory now has only two jars:

```
$ ls -1 examples/target/scala-2.11/jars/
scopt_2.11-3.3.0.jar
spark-examples_2.11-2.0.0-SNAPSHOT.jar
```

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #12544 from vanzin/SPARK-14744.