downgrading WARNING to DEBUG to avoid spamming (see ATLASRECTS-5086)

As in ATLASRECTS-5086 and ATLASRECTS-4582 it is discussed that WARNINGs are being triggered due to changes in the underlying track collection being passed to the InDetRecStatistics alg (some collections no longer have summaries associated) an are relatively benign. The underlying problem will be looked into, but in the meantime, this MR will stop these messages from flooding the log files.