Delete duplicate rename function in ProjectManager

#### What I did
Got rid of a rename function that was an almost exact duplicate of another. In the sake of "single responsibility per method", I removed it and adopted the code as necessary.

#### Discussion
Currently, just like before, the project is not saved if only the description was changed and thus possibly lost. Is this on purpose, did I miss a "hidden" call to `saveProject()` (like `renameProject()` has) or is this just a plain mistakw? There's a `saveProject()` call in `MainMenuActivity#onPause()`, but I'm not sure if that can be trusted. 

i.e. I just trashed my local copy of the Hanabi program, by loading it, leaving Catroid and killing it with the Android recent apps screen (swiped it away), making me think that an AsyncTask is probably not suitable for what we do with it. We probably need a Service. But that's a completely different issue which I am not even addressing here.

#### Relevant links
[Diff of both functions before the changes](https://gist.github.com/aried3r/401ac00d65e12ebc6d12/revisions)
[Testrun](https://jenkins.catrob.at/view/All-Categories/view/Catroid/job/Catroid-Multi-Job-Custom-Branch/708/)