Remove dashed border on topic-voting buttons

This was a bit confusing with the topic icons using a dashed blue border
for "missing icon". Now that the "already used" buttons are far more
distinct, this isn't necessary any more anyway.

tildes/scss/modules/_topic.scss

@@ -134,10 +134,6 @@
   height: auto;
   min-width: 3rem;
 
-  &.btn {
-    border-style: dashed;
-  }
-
   &.btn-used {
     background-color: darken($violet, 3%);
     border-color: darken($violet, 8%);