Simplify mouse-dragging implementation.

Simplify mouse-dragging implementation.
Now that DEL deletes active regions, we can handle it by using the
ordinary region instead of a separate overlay.

* mouse.el (mouse-drag-overlay): Variable deleted.
(mouse-move-drag-overlay, mouse-show-mark): Functions deleted.
(mouse--remap-link-click-p): New function.
(mouse-drag-track): Handle dragging by using temporary Transient
Mark mode, instead of a special overlay.
(mouse-kill-ring-save, mouse-save-then-kill): Don't call
mouse-show-mark.

* mouse-sel.el (mouse-sel-selection-alist): mouse-drag-overlay
deleted.