Debug log is filled with USER_INFO

Ok, I had the same error, and after much digging, may have a solution.

In the debug log area, delete all logging levels and all traces.

Then in the dev console, do this query (using the tooling API):

SELECT Id FROM TraceFlag

Delete all the resulting rows. There was one row that kept regenerating (probably because I was in the dev console and it corresponded to that).

This fixed the issue for me.