Update removefolder.php

Thanks to the help of Chris D, we determined the real issue is that some redundant code is in this file for FTP but that screen has been removed during the installation.

edit (by @zero-24)

### Testinstructions

Install Joomla on two webservers, one requiring FTP to be used, the other not requiring FTP to be used.
Upon completion in both situations:

- the installation folder must be removed without problem.
- Verify content of robots.txt, unchanged if existing before installation, otherwise equal to content of robots.txt.dist.
- no joomla.xml may exist in root folder.