update: Drop redundant bp map updates and functions

update: Drop redundant bp map updates and functions

produce_sect() updates the bp map several times.  This is wasteful:
since only ship, plane and land unit building reads it, bp map writes
before the last one are never read.  Update it just once for every
sector.

The update for sectors that are stopped or whose owner is broke is the
only remaining use of bp_put_items().  Since available work must still
be unchanged there, we can replace it by bp_set_from_sect().

bp_get_item(), bp_put_item(), bp_get_items(), bp_get_avail() and
bp_put_avail() are now unused.  Drop them.

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