jstat prints debug message when debugging is disabled

jstat is designed to handle the case where a perf counter doesn't exist. In the output jstat will then print a "-" instead of the perf counter value to indicate that the counter doesn't exist. However, jstat also prints an unnecessary message, which clobbers the output. This message might still be useful for debugging, but should only be printed when jstat is running with debug messages enabled (-J-DExpressionResolver.debug=true).