bc: G.sbgn and G.send are unused, remove them

bc: G.sbgn and G.send are unused, remove them

function                                             old     new   delta
bc_program_index                                      66      64      -2
bc_parse_expr_empty_ok                              1840    1838      -2
dc_main                                              176     168      -8
bc_main                                               73      65      -8
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/4 up/down: 0/-20)             Total: -20 bytes
   text	   data	    bss	    dec	    hex	filename
 980327	    485	   7296	 988108	  f13cc	busybox_old
 980307	    485	   7296	 988088	  f13b8	busybox_unstripped

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