bc: BC_STATUS_VEC_ITEM_EXISTS is not a real error code, its message was never used

bc: BC_STATUS_VEC_ITEM_EXISTS is not a real error code, its message was never used

It was only used to indicate rusult of bc_map_insert() - did we insert, or
did we find that this key is already in the map?

function                                             old     new   delta
bc_map_insert                                        142     145      +3
bc_program_addFunc                                   226     225      -1
bc_err_msgs                                          184     180      -4
bc_program_search                                    152     143      -9
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/3 up/down: 3/-14)             Total: -11 bytes
   text	   data	    bss	    dec	    hex	filename
 987904	    485	   7296	 995685	  f3165	busybox_old
 987873	    485	   7296	 995654	  f3146	busybox_unstripped

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>