Use plain epoch millis instead of DateTime

JodaTime is not strictly needed for comparing epoch times and
is not included anymore on Gerrit master (2.16/3.0).

Using a pure long timestamp comparison resolves compile problems
with master and makes it compatible with more versions of Gerrit.

Change-Id: Ia78ebd352b47ed48205bbd4533e6dfde1cc09d5b

src/main/java/com/googlesource/gerrit/plugins/websession/flatfile/FlatFileWebSessionCache.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/websession/flatfile/FlatFileWebSessionCache.java b/src/main/java/com/googlesource/gerrit/plugins/websession/flatfile/FlatFileWebSessionCache.java
index 50acdf0..a240aee 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/websession/flatfile/FlatFileWebSessionCache.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/websession/flatfile/FlatFileWebSessionCache.java
@@ -23,7 +23,6 @@
 import com.google.inject.Inject;
 import com.google.inject.Singleton;
 
-import org.joda.time.DateTime;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -83,8 +82,8 @@
   public void cleanUp() {
     for (Path path : listFiles()) {
       Val val = readFile(path);
-      DateTime expires = new DateTime(val.getExpiresAt());
-      if (expires.isBefore(new DateTime())) {
+      long expires = val.getExpiresAt();
+      if (expires < System.currentTimeMillis()) {
         deleteFile(path);
       }
     }