Use list-style: none in comment emails

Aggressive quoting in Inbox garbles HTML comment emails where sections
are quoted by creating additional UL > LI > UL > LI structures. This
results in extraneous list bullets. Remove list bullets altogether for a
cleaner look.

Change-Id: I3ca09b0820cbeb6e11056049cb6e41999781d42b

gerrit-server/src/main/resources/com/google/gerrit/server/mail/CommentHtml.soy

@@ -39,6 +39,7 @@
   {/let}
 
   {let $ulStyle kind="css"}
+    list-style: none;
     padding-left: 20px;
   {/let}