substitute: Don't fetch /nix-cache-info.

substitute: Don't fetch /nix-cache-info.

This avoids one GET request every time 'fetch-narinfos' is called.
The file itself was essentially useless.

* guix/scripts/substitute.scm (<cache-info>, download-cache-info):
Remove.
(%unreachable-hosts): New variable.
(open-connection-for-uri/maybe): New procedure.
(fetch-narinfos)[handle-narinfo-response]: Check whether NARINFO has its
'path' under (%store-prefix) and ignore it otherwise.  Move
'update-progress!' call before 'if'.
[do-fetch]: Remove 'port' parameter.  Use
'open-connection-for-uri/maybe'.
Remove call to 'download-cache-info'.