Don't need to erase set as destructors are already called in ~ofOpenALSoundPlayer()

Calling 
```
players.erase(this);
```
causes the pointer in question to be destroyed (deleted), so exiting the destructor of the ofOpenALSoundPlayer causes a segmentation fault.

As the ~ofOpenALSoundPlayer() is already being called, it's fine to let ~set() do the rest of the clean up.