Remove double dashes marking end of args in 'purge' command

Without this change, the apt-get remove command executed from ```ceph-deploy uninstall``` or ```ceph-deploy purge``` will be:

```
sudo apt-get -q remove -f -y --force-yes -- ceph ceph-mds ceph-common ceph-fs-common
```

The double dashes ('--') will however prevent apt-get from correctly parsing the options, resulting in this error:

```
Command line option 'f' [from -f] is not known.
```

The same can be easily observed with a simple example:

```
$ sudo apt-get -q remove --force-yes -- vim
E: Command line option --force-yes is not understood
```

```
$ sudo apt-get -q remove --force-yes vim
Reading package lists...
Building dependency tree...
Reading state information...
[and the usual good stuff]
```

I don't see any need for the double dashes here (I hope I am not overlooking something :)).