Remove non-mission land unit reaction

Land unit reactions are overly complex because we have two different
concepts controlling them: reaction radius (set with lrange) and
reserve mission (set with mission).  You need to deal with both to set
up or query reactions.

Commit 8d0e1af5 "fixed" this by making reserve missions meaningless.

The previous commit made reserve missions meaningful again: they
support an op-area now.  This brought back the problem of having to
deal with two separate commands to accomplish one thing.

Fix this for good by removing non-mission land unit reaction
alltogether.  The only feature we lose by that is the ability to order
land units to react until the order is explicitely cancelled.  That's
because missions are implicitely cleared by many commands and events,
while non-mission reaction wasn't.  Closes #858121 and #858122.

Remove the non-mission reaction case from att_reacting_units().

Don't limit reserve missions to the land unit's reaction radius: make
lnd_reaction_range() return the type's maximum radius instead of
lnd_rad_max.

The reaction radius is now useless.  Remove the lrange command, and
struct lndstr member lnd_rad_max along with its selector react.
Remove land command's column rd.  Make ldump show column react as
zero.  Deprecate edit key 'P' in dounit(), and don't show it in
pr_land().

info/mission.t

@@ -67,7 +67,7 @@ Some missions require the designation of an 'op sector', short for operations
 sector. This is the center of an area that the mission is focused on.
 The op sector may be any sector that is within the unit's range.
 (Firing range for ships & land units on interdiction, reaction range
-for land units on reserve, changeable with lrange command, flying
+for land units on reserve, flying
 range for planes, changeable with range command).
 The unit will exert influence in a radius around the op sector.
 By default, the radius will be as large as possible, i.e. the biggest range