String#delete_suffix!

static VALUE
rb_str_delete_suffix_bang(VALUE str, VALUE suffix)
{
    rb_len_t suffixlen;
    str_modifiable(str);

    suffixlen = deleted_suffix_length(str, suffix);
    if (suffixlen <= 0) return Qnil;

    return str_shrink(str, RSTRING_LEN(str) - suffixlen);
}