Button: Remove obsolete mouse click coordinates checking. Fixed #7665 - Radio button & checkboxes ignore mouseclicks for minor mouse movements

I've had someone asking about [jQuery UI's ticket 7665](http://bugs.jqueryui.com/ticket/7665) not long ago, so here's my recap of its history thus far:

> As I've explained [here](http://bugs.jqueryui.com/ticket/7665#comment:11), the [#6970](http://bugs.jqueryui.com/ticket/6970) bug fix introduced this code to completely prevent users from drag-clicking, instead of actually fixing the widget to work with drag-clicks. IMO this was a bad decision as it harms usability (mostly touch and high DPI input devices).

> My fix for [#5518](http://bugs.jqueryui.com/ticket/5518) did take care of inconsistent double and drag-clicks, making the coordinates checking code obsolete.

> Then, I've tried to push the [PR 945](https://github.com/jquery/jquery-ui/pull/945) to remove all that obsolete code, but it has been stagnated ever since.

This PR is less audacious than my previous one in that instead of re-creating the whole clicking functionality, it simply removes obsolete  code leaving the drag/click handling to the browser's default behavior.

Passes all current tests. Haven't added any new test because I believe this kind of browser behavior cannot be simulated.