Add Checkstyle UnusedImports

Author: garydgregory

src/conf/checkstyle.xml

@@ -74,6 +74,7 @@ limitations under the License.
     <module name="RedundantImport" />
     <module name="RedundantModifier" />
     <module name="RightCurly" />
+    <module name="UnusedImports" />
     <module name="WhitespaceAfter" />
     <module name="WhitespaceAround" />
   </module>