Remove dev symlinks before deleting Release or Debug dirs

@redmunds @peterflynn 

Fix a data deletion error in the following scenario:

1. Setup/build brackets-shell on Windows
2. Run the `setup_for_hacking` script, passing the `Release` or `Debug` directory from step 1
3. Run the `scripts/make_symlinks.bat` script, either explicitly, or implicitly by running `scripts/setup.sh` when CEF needs to be downloaded.

Results: the contents of the symlinked `dev` directory are deleted.

To work around this, explicitly remove the symlinks to `dev` before removing the `Release` and `Debug` directories.