removed an orphaned submodule

There is a leftover submodule which causes the following to fail:
```
$ git submodule status
No submodule mapping found in .gitmodules for path 'labs/architecture-examples/plastronjs/js/lib/PlastronJS'
```
Since SourceTree uses this by default when pulling, it causes an error on each pull.

This pull request removes the orphaned submodule.