String#delete_prefix!

static VALUE
rb_str_delete_prefix_bang(VALUE str, VALUE prefix)
{
    rb_len_t prefixlen;
    str_modify_keep_cr(str);

    prefixlen = deleted_prefix_length(str, prefix);
    if (prefixlen <= 0) return Qnil;

    return rb_str_drop_bytes(str, prefixlen);
}