bc: shrink "dc only" code by dropping unused structure members

bc: shrink "dc only" code by dropping unused structure members

function                                             old     new   delta
zbc_program_num                                      979     981      +2
zbc_program_pushArray                                147     145      -2
bc_program_pushVar                                   198     196      -2
bc_program_func                                       17      15      -2
dc_num_printChar                                      24      21      -3
bc_program_retire                                     35      32      -3
bc_program_binOpRetire                                46      43      -3
zdc_program_printStream                              153     148      -5
zbc_program_prep                                      91      86      -5
zbc_program_copyToVar                                300     295      -5
zdc_program_modexp                                   721     715      -6
zbc_program_binOpPrep                                311     305      -6
bc_program_addFunc                                   138     132      -6
bc_num_printNewline                                   51      45      -6
bc_num_printHex                                       67      61      -6
bc_num_printDigits                                   137     131      -6
zdc_program_assignStr                                146     137      -9
bc_program_reset                                      64      55      -9
bc_func_free                                          27       5     -22
bc_parse_free                                         38       8     -30
bc_parse_create                                       92      47     -45
bc_func_init                                          50       5     -45
dc_main                                              691     645     -46
zdc_program_execStr                                  496     442     -54
zbc_program_print                                    677     623     -54
zbc_vm_process                                       204     137     -67
zbc_program_exec                                    4132    4057     -75
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/26 up/down: 2/-522)          Total: -520 bytes
   text	   data	    bss	    dec	    hex	filename
 969767	    485	   7296	 977548	  eea8c	busybox_old
 969210	    485	   7296	 976991	  ee85f	busybox_unstripped

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