Fix issues reported by linter

Author: Pushkar-Mahajan4

core/src/main/java/hudson/os/WindowsUtil.java

@@ -29,7 +29,6 @@
 import hudson.Functions;
 import java.io.File;
 import java.io.IOException;
-import java.nio.charset.Charset;
 import java.util.Arrays;
 import java.util.regex.Pattern;
 import java.util.stream.Collectors;