bug#6701: 24.0.50; Cut and paste with mouse-3/mouse-2 no longer works

[Chong Yidong]

I've checked a fix into the trunk.  Now double-click mouse-3 should
correctly place the text into the primary.

Note to David: the problem was fixed in a somewhat simpler manner than
we discussed.  We just had to call deactivate-mark in
mouse-save-then-kill before killing, in order to freeze off a copy of
the primary selection.