Clean up redundant code constructs

Results of the following IntelliJ inspections:
 * Redundant 'isInstance' or 'cast' call
 * Redundant array creation
 * Redundant Collection operation
 * Redundant String operation
 * Redundant type cast

The only manual change was to remove a suppression and comment about a
redundant cast which doesn't appear to be an issue in the current
version of JDK 8

Change-Id: I34a70d436512cc4efe9dafb0917be3dfce4ba06b

java/com/google/gerrit/mail/MailHeaderParser.java

@@ -103,6 +103,6 @@ public class MailHeaderParser {
   }
 
   private static String extractFooter(String key, String line) {
-    return line.substring(line.indexOf(key) + key.length(), line.length()).trim();
+    return line.substring(line.indexOf(key) + key.length()).trim();
   }
 }