The presence of a close button you can left-click does not replace the desirable
feature to be able to middle-click on a window to close it. With this, the whole
window is a click target, so it can be much much faster than having to aim for the
little close button. Also it's off by default, so a user who goes out of their way
to turn it on is signaling that they want to accept the risk of accidentally closing
a window by accident.
Finally, the feature is only enabled for middle-click, so people can never accidentally
wreck Present Windows for themselves by assigning it to left-click by accident.
This reverts commit 55585514f926d1251148e876bfe9ce3504432997.