metalink: remove

metalink: remove

Warning: this will make existing curl command lines that use metalink to
stop working.

Reasons for removal:

1. We've found several security problems and issues involving the
   metalink support in curl. The issues are not detailed here. When
   working on those, it become apparent to the team that several of the
   problems are due to the system design, metalink library API and what
   the metalink RFC says. They are very hard to fix on the curl side
   only.

2. The metalink usage with curl was only very briefly documented and was
   not following the "normal" curl usage pattern in several ways, making
   it surprising and non-intuitive which could lead to further security
   issues.

3. The metalink library was last updated 6 years ago and wasn't so
   active the years before that either. An unmaintained library means
   there's a security problem waiting to happen. This is probably reason
   enough.

4. Metalink requires an XML parsing library, which is complex code (even
   the smaller alternatives) and to this day often gets security
   updates.

5. Metalink is not a widely used curl feature. In the 2020 curl user
   survey, only 1.4% of the responders said that they'd are using it. In
   2021 that number was 1.2%. Searching the web also show very few
   traces of it being used, even with other tools.

6. The torrent format and associated technology clearly won for
   downloading large files from multiple sources in parallel.

Cloes #7176