[GR-16616] Check unused imports with CheckStyle

* Which provides a fast way to check locally with "mx checkstyle".

Author: eregon

graalpython/com.oracle.graal.python/.checkstyle_checks.xml

@@ -34,6 +34,7 @@
       <property name="format" value="^[_a-zA-Z][_a-zA-Z0-9]*$"/>
     </module>
     <module name="RedundantImport"/>
+    <module name="UnusedImports"/>
     <module name="LineLength">
       <property name="max" value="300"/>
     </module>