Remove deprecated from_target usage in examples.

This eliminates a deprecation warning that happens during full graph parse:

```
[illuminati pants (master)]$ ./pants dependees
WARN] While parsing BuildFile(examples/src/protobuf/org/pantsbuild/example/unpacked_jars/BUILD, FileSystemProjectTree(/Users/kwilson/dev/pants)):
    3:
    4: java_protobuf_library(name='unpacked_jars',
    5:   # This uses the legacy deferred sources mechanism.
*   6:   sources=from_target(':external-source'),
        DeprecationWarning: DEPRECATED: pants.build_graph.from_target.__call__ will be removed in version 1.3.0.
  Using sources = from_target() has been deprecated. Try using remote_sources() instead.

For example, instead of this:

  java_protobuf_library(name='proto',
    sources=from_target(':other-target'),
    platform='java7',
  )

Try this:

  remote_sources(name='proto',
    dest=java_protobuf_library,
    sources_target=':other-target',
    args=dict(
      platform='java7',
    )
  )

    7: )
    8:
    9: remote_sources(name='better-unpacked-jars',
   10:   # This uses the new deferred sources mechanism.

```

Testing Done:
CI is green here: https://travis-ci.org/pantsbuild/pants/builds/162339404

Bugs closed: 3900, 3901

Reviewed at https://rbcommons.com/s/twitter/r/4262/

examples/src/protobuf/org/pantsbuild/example/unpacked_jars/BUILD

@@ -1,13 +1,7 @@
 # Copyright 2014 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-java_protobuf_library(name='unpacked_jars',
-  # This uses the legacy deferred sources mechanism.
-  sources=from_target(':external-source'),
-)
-
-remote_sources(name='better-unpacked-jars',
-  # This uses the new deferred sources mechanism.
+remote_sources(name='unpacked_jars',
   dest=java_protobuf_library,
   sources_target=':external-source',
 )