convert: Drop broken code to charge security unit mobility

convert: Drop broken code to charge security unit mobility

Conversion is easier when land units with capability security are
present.  Each such land unit is charged 10 mobility.  The mobility
charge is undocumented.

Land unit mobility is charged even when conversion turns out to be
impossible, say because the sector has no mobility.  I call this a
bug.  Has been that way since security land units were added in
Chainsaw 3.

Except the mobility charge doesn't actually work anymore: the changed
land unit is never written back.  Broken in commit 82c9166, v4.3.16.
Fix this bug would be trivial, but would bring back the bug described
above, and fixing that one is harder, and doesn't feel worthwhile.

Remove the broken charging of land unit mobility instead.

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