remove target_acknowledge_created_inferior

remove target_acknowledge_created_inferior

This target_ops routine appears to be unused in practice, so this patch
removes it.

gdb/ChangeLog:

        * inf-child.c (inf_child_acknowledge_created_inferior): Delete.
        (inf_child_target): Do not set t->to_acknowledge_created_inferior.
        * inf-ptrace.c (inf_ptrace_create_inferior): Remove call to
        target_acknowledge_created_inferior.
        * inf-ttrace.c (inf_ttrace_him): Likewise.
        * target.h (struct target_ops): Delete to_acknowledge_created_inferior
        field.
        (target_acknowledge_created_inferior): Delete.
        * target.c (update_current_target): Remove setting of
        to_acknowledge_created_inferior field. Do default value for that
        field either.
        (debug_to_acknowledge_created_inferior): Delete.
        (setup_target_debug): Remove setting of
        current_target.to_acknowledge_created_inferior.