Replace Guava's Optional with java.util.Optional

Change-Id: I645c81e7b1091f70b4d9020af68dffdc56603974

gitiles-servlet/src/main/java/com/google/gitiles/ArchiveServlet.java

diff --git a/gitiles-servlet/src/main/java/com/google/gitiles/ArchiveServlet.java b/gitiles-servlet/src/main/java/com/google/gitiles/ArchiveServlet.java
index f004c9f..402674c 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/ArchiveServlet.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/ArchiveServlet.java
@@ -17,9 +17,9 @@
 import static javax.servlet.http.HttpServletResponse.SC_NOT_FOUND;
 import static javax.servlet.http.HttpServletResponse.SC_OK;
 
-import com.google.common.base.Optional;
 import com.google.common.base.Strings;
 import java.io.IOException;
+import java.util.Optional;
 import javax.servlet.ServletException;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;