Consider removing the directory selection from WCFSetup

[TimWolla]

The directory selection is problematic for little benefit (the vast majority of users won't change anything at all): It starts to deploy the files before it is verified that the WCFSetup will be able to complete successfully to some reasonable degree of certainty, as the database credentials have not yet been validated. By removing the selection, the installation directory will be known a-priori, so that the database step knows where to deploy the config.inc.php without carrying along all the database credentials in a hidden input.