Awful completion buffer behaviour

[Po Lu]

Pressing tab in the minibuffer when a completions buffer is visible used
to scroll the window containing that buffer.

In Emacs 29, it selects that window instead, and moves point to the next
visible completion.

That behaviour is horrid, and even worse, I didn't find a NEWS entry
detailing how to turn it off (it should be off by default anyway).

Please remove the offending change or put it behind a switch.  Thanks.