bc: delete unused (write-only) BcParse::nbraces member

bc: delete unused (write-only) BcParse::nbraces member

function                                             old     new   delta
zbc_lex_next                                        2296    2309     +13
bc_parse_expr_empty_ok                              2021    2025      +4
bc_vm_init                                           760     757      -3
bc_num_printNewline                                   54      51      -3
zbc_num_divmod                                       156     150      -6
bc_parse_reset                                       113     106      -7
zbc_lex_number                                       200     192      -8
bc_parse_number                                       83      66     -17
zdc_parse_expr                                       707     671     -36
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/7 up/down: 17/-80)            Total: -63 bytes
   text	   data	    bss	    dec	    hex	filename
 982275	    485	   7296	 990056	  f1b68	busybox_old
 982212	    485	   7296	 989993	  f1b29	busybox_unstripped

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