You disabled php exec functions in your php.ini for php cli (php schell scripts) which causes ispconfig to fail. The changes in the php.ini cause also the uninstall script to fail. Edit your cli php.ini file, search for the "disable_functions" line and comment it out by adding a # in front of the line. Now ispconfig will work again and you can also uninstall ispconfig if you still want to do that now.
