bc: "dc only" config does not need G.prog.fn_map

bc: "dc only" config does not need G.prog.fn_map

function                                             old     new   delta
bc_program_add_fn                                      -      43     +43
bc_vm_init                                           655     675     +20
bc_program_addFunc                                   138     118     -20
zdc_parse_expr                                       658     635     -23
zdc_program_asciify                                  447     407     -40
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 1/3 up/down: 63/-83)            Total: -20 bytes
   text	   data	    bss	    dec	    hex	filename
 984739	    489	   7312	 992540	  f251c	busybox_old
 984712	    489	   7312	 992513	  f2501	busybox_unstripped

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