Mobile comment collapse button: remove left border

This looked a bit off because the comment always has its own left
border, so the border was 1px thicker next to the button (and sometimes
a different color).

tildes/scss/modules/_btn.scss

@@ -38,6 +38,10 @@
 
   font-weight: normal;
 
+  @media (max-width: $size-md) {
+    border-left: 0;
+  }
+
   margin-right: 0.4rem;
   @media (min-width: $size-md) {
     margin-right: 0.2rem;