FIX: Do not cut off long polls (#12854)

Some polls with images can be very long. Those which showed a pie chart
for the results had a fixed height set, which meant that some long polls
could be cut off.

plugins/poll/assets/stylesheets/common/poll.scss

@@ -161,9 +161,6 @@ div.poll {
 div.poll.pie {
   .poll-container {
     display: inline-block;
-    height: 340px;
-    max-height: 340px;
-    overflow-y: auto;
   }
   .poll-info {
     display: inline-block;