curl: stop using metalink
curl: stop using metalink

The metalink feature was removed in cURL v7.78.0 due to security
concerns.

Git for Windows never used it, it was a feature designed to pick the
best mirror for a given website, and there is no support for it in Git.
It only affects `curl.exe`. For more details, see
https://github.com/curl/curl/blob/70233523976f~2/docs/cmdline-opts/metalink.d

Signed-off-by: Johannes Schindelin <53fb8db7833fca1e1746dd8592f61048d350f64c@gmx.de>

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