Remove unused import in WebhookServlet

Change-Id: I4c28a926530cede47d64ed1e50cedfcd6ff298ca

github-plugin/src/main/java/com/googlesource/gerrit/plugins/github/notification/WebhookServlet.java

diff --git a/github-plugin/src/main/java/com/googlesource/gerrit/plugins/github/notification/WebhookServlet.java b/github-plugin/src/main/java/com/googlesource/gerrit/plugins/github/notification/WebhookServlet.java
index 69819fd..fa746e2 100644
--- a/github-plugin/src/main/java/com/googlesource/gerrit/plugins/github/notification/WebhookServlet.java
+++ b/github-plugin/src/main/java/com/googlesource/gerrit/plugins/github/notification/WebhookServlet.java
@@ -26,7 +26,6 @@
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
 import java.util.Arrays;
-import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;