[SPARK-4291][Build] Rename network module projects

The names of the recently introduced network modules are inconsistent with those of the other modules in the project. We should just drop the "Code" suffix since it doesn't sacrifice any meaning, especially before they get into an official release.

```
[INFO] Reactor Build Order:
[INFO]
[INFO] Spark Project Parent POM
[INFO] Spark Project Common Network Code
[INFO] Spark Project Shuffle Streaming Service Code
[INFO] Spark Project Core
[INFO] Spark Project Bagel
[INFO] Spark Project GraphX
[INFO] Spark Project Streaming
[INFO] Spark Project Catalyst
[INFO] Spark Project SQL
[INFO] Spark Project ML Library
[INFO] Spark Project Tools
[INFO] Spark Project Hive
[INFO] Spark Project REPL
[INFO] Spark Project YARN Parent POM
[INFO] Spark Project YARN Stable API
[INFO] Spark Project Assembly
[INFO] Spark Project External Twitter
[INFO] Spark Project External Kafka
[INFO] Spark Project External Flume Sink
[INFO] Spark Project External Flume
[INFO] Spark Project External ZeroMQ
[INFO] Spark Project External MQTT
[INFO] Spark Project Examples
[INFO] Spark Project Yarn Shuffle Service Code
```

Author: Andrew Or <andrew@databricks.com>

Closes #3148 from andrewor14/build-drop-code and squashes the following commits:

eac839b [Andrew Or] Network -> Networking
d01ad47 [Andrew Or] Rename network module project names

(cherry picked from commit 7afc8564f33eb2868f458f85046f59a51b516ed6)
Signed-off-by: Patrick Wendell <pwendell@gmail.com>