fire: Fix artillery splashing the bridge span under itself

fire: Fix artillery splashing the bridge span under itself

multifire() writes back the firing sector after applying damage.  When
an artillery unit on a bridge span commits suicide by shelling down
the supporting bridge heads, this writeback puts the bridge span right
back (less the land units and planes on it), triggering a seqno oops.
On the next update of the bridge head, the bridge span falls again.
Broken in commit fe5b266, v4.3.14.

The problematic write back is superfluous.  Remove it along with a few
equally superfluous ones.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>