o.c.archive.reader.rdb: Lower log level for severity mapping message.

SNS Oracle RDB has extra "Unused" severity table entry. It's mapped to
"UNDEFINED" severity and - duh - not used, but kept showing up in the
log...
Merge pull request #236 from ControlSystemStudio/bug_233_misleading_warning

o.c.archive.reader.rdb: Lower log level for severity mapping message.: Fix #233