bc: remove "ready for more input" message, GNU bc does not print that

bc: remove "ready for more input" message, GNU bc does not print that

function                                             old     new   delta
bc_read_line                                         285     268     -17
bc_vm_run                                            684     630     -54
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-71)             Total: -71 bytes

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