Remove URL related classes from supported javalib

[tanishiking]

Close #2785

Those classes (URL, URLConnection, and URLClassLoader) are already removed in #2788 Let's close the original issue by removing them from the list of supported classes in the doc.

(While I'm not sure this list is well maintained + is it useful for scala-native users to lookup 🤔 )

[LeeTibbert]

@tanishiking

Thank you for finding and fixing this.

While I'm not sure this list is well maintained

The list is maintained, and with some effort and hope of correctness. This PR is
part of that effort.

Since the maintenance is manual, not automatic, it can be called "pretty-well" maintained but,
sigh, it is hard to call it "well maintained".

For years there was an automatically generated, then manually curated list. That fell into
disrepair and the automatic part generated many internal classes, making it hard even
for experts to understand.

is it useful for scala-native users to lookup

javalib is a subset of the JDK and will be for a long_time/forever. Its growth and
development has been organic. One can not say "Everything in Tier 1 is implemented;
everything else is not".

I believe that the list is an essential part of a "Say what we do and do what
we say" approach. Yes, it is an extra step, both in creating and in reviewing a PR. There are also those
who say "the code is the documentation". The SN project gains more merit and good energy from letting users
discover up front what is and what is not implemented and working than from catching queries from
hostile Internet users "What does this missing symbol mean?" or "Why does this class not work?".

I am one of the people who read and review PRs as they come across the wire. I try to catch
doc implications, especially to javalib & posixlib. Looks like I missed this one. I appreciate
that you caught it.

[LeeTibbert]

(While I'm not sure this list is well maintained + is it useful for scala-native users to lookup 🤔 )

Please do keep asking questions such as these. I am of the school which says that if a feature can
not find a champion, and preferably 2 or many, then it should be removed. There is a great book,
now old, called "The Design of Everyday Things" which has influenced me.