basic/in-addr-util: drop check for prefix length in formatting function

basic/in-addr-util: drop check for prefix length in formatting function

The general rule should be to be strict when parsing data, but lenient
when printing it. Or in other words, we should verify data in verification
functions, but not when printing things. It doesn't make sense to refuse
to print a value that we are using internally.

We were tripping ourselves in some of the print functions:
we want to report than an address was configured with too-long prefix, but
the log line would use "n/a" if the prefix was too long. This is not useful.

Most of the time, the removal of the check doesn't make any difference,
because we verified the prefix length on input.