Fix checkstyle in OMMetrics.java

Change-Id: Ib16889a190f114d19d95b628430d2cd78604fd84

Author: jojochuang

hadoop-ozone/ozone-manager/src/main/java/org/apache/hadoop/ozone/om/OMMetrics.java

@@ -29,7 +29,6 @@
 import org.apache.hadoop.metrics2.lib.DefaultMetricsSystem;
 import org.apache.hadoop.metrics2.lib.MutableCounterLong;
 import org.apache.hadoop.metrics2.lib.MutableGaugeInt;
-import org.apache.hadoop.ozone.om.OMConfigKeys;
 import org.apache.hadoop.ozone.om.snapshot.OMSnapshotDirectoryMetrics;
 import org.apache.hadoop.util.Time;