Use try-with-resource for ReviewDb

As ReviewDb is auto-closable, use this.

Change-Id: I2929ec873ef20dbf49c67bd2c7b84b17e7559781
diff --git a/src/main/java/com/googlesource/gerrit/plugins/serviceuser/ServiceUserResolver.java b/src/main/java/com/googlesource/gerrit/plugins/serviceuser/ServiceUserResolver.java
index 155cada..25d6241 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/serviceuser/ServiceUserResolver.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/serviceuser/ServiceUserResolver.java
@@ -98,8 +98,7 @@
       return Collections.emptyList();
     }
 
-    final ReviewDb db = schema.open();
-    try {
+    try (ReviewDb db = schema.open()) {
       RequestContext context = new RequestContext() {
         @Override
         public CurrentUser getUser() {
@@ -153,8 +152,6 @@
       } finally {
         tl.setContext(old);
       }
-    } finally {
-      db.close();
     }
   }