autoconfig.php only to be deleted on successful installation


fixes #12277

@jnfrmarks 
Can we please use single line comments instead of wasting three lines for such things? - Or at least agree on one common standard... - Currently everybody uses whatever he likes.

I personally hate the three line waste and it makes it harder to me to see on the first view how many variables are there etc....

That said: PHPStorm also supports single line comments by just typing `/** `.