Fix /buttons buttons not being displayed

- On long popup texts, the buttons of the `/popup` slash command will not be displayed, cause by an unneeded overflow CSS property

Fixes #3654

public/css/scrollable-button.css

@@ -1,6 +1,5 @@
 .scrollable-buttons-container {
     max-height: 50vh;  /* Use viewport height instead of fixed pixels */
-    overflow-y: auto;
     -webkit-overflow-scrolling: touch; /* Momentum scrolling on iOS */
     margin-top: 1rem; /* m-t-1 is equivalent to margin-top: 1rem; */
     flex-shrink: 1;