Just trying to delete all the documents, and did this:
http://localhost:8983/solr/update?stream.body=%3Cdelete%3E%3Cquery%3E*:*%3C/query%3E%3C/delete%3E
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Not sure if it matters but you might encode the : too
http://localhost:8983/solr/update?stream.body=%3Cdelete%3E%3Cquery%3E*%3A*%3C%2Fquery%3E%3C%2Fdelete%3E
Another thing to try is to use the POST method (the preferred way to call update):
curl http://localhost:8983/solr/update?commit=true -H "Content-Type: text/xml" --data-binary '*:* '