Set the region according to CLICK; the second time, kill it.
CLICK should be a mouse click event.
If the region is inactive, activate it temporarily. Set mark at
the original point, and move point to the position of CLICK.
If the region is already active, adjust it. Normally, do this by
moving point or mark, whichever is closer, to CLICK. But if you
have selected whole words or lines, move point or mark to the
word or line boundary closest to CLICK instead.
If `mouse-drag-copy-region' is non-nil, this command also saves the
new region to the kill ring (replacing the previous kill if the
previous region was just saved to the kill ring).
If this command is called a second consecutive time with the same
CLICK position, kill the region (or delete it if `mouse-drag-copy-region' is non-nil) (fn CLICK)