Don't beep when plane, land unit or nuke die on collapsing bridge

Don't beep when plane, land unit or nuke die on collapsing bridge

Not nice, because it could beep many times, and could put beeps in
bulletins.  Moreover, it misused mpr() and thus put the beep in its
own bulletin.  The read command normally merges this bulletin with the
adjacent ones, but if the bulletins are more than five seconds apart
(clock jumped somehow), we can get an empty bulletin just for the
beep.

Beeping was added in v4.0.18.