Revert "Add a "Back to top" button on mobile"

This reverts commit 630ed4467f.

tildes/scss/_layout.scss

@@ -51,19 +51,6 @@ body {
   }
 }
 
-#back-to-top-buffer {
-  // This is an invisible "buffer" for the "Back to top" button. It uses the
-  // Intersection Observer API to detect when the entire buffer has been
-  // scrolled off the screen, and will make the button visible at that point.
-  // So to have the button show up when the user has scrolled down at least two
-  // viewport heights, we set its height at 200vh.
-  height: 200vh;
-  position: absolute;
-  top: 0;
-  right: 0;
-  width: 1px;
-}
-
 #site-header {
   grid-area: header;
 

tildes/scss/modules/_btn.scss

@@ -68,22 +68,3 @@
     border: 1px solid;
   }
 }
-
-.btn-back-to-top {
-  position: fixed;
-  bottom: 0.4rem;
-  right: 0.4rem;
-
-  font-size: 0.5rem;
-
-  opacity: 0;
-  visibility: hidden;
-  transition: all 0.5s ease;
-
-  &.btn-back-to-top-visible {
-    @media (max-width: $size-md) {
-      opacity: 1;
-      visibility: visible;
-    }
-  }
-}

tildes/scss/themes/_theme_base.scss

@@ -111,14 +111,6 @@
     }
   }
 
-  .btn-back-to-top {
-    background-color: map-get($theme, "background-primary");
-
-    &:visited {
-      color: map-get($theme, "link");
-    }
-  }
-
   .btn-comment-collapse {
     color: map-get($theme, "foreground-secondary");
     border-color: map-get($theme, "border");

tildes/static/js/behaviors/back-to-top-buffer.js

@@ -1,20 +0,0 @@
-// Copyright (c) 2019 Tildes contributors <code@tildes.net>
-// SPDX-License-Identifier: AGPL-3.0-or-later
-
-$.onmount('[data-js-back-to-top-buffer]', function() {
-    // Uses the Intersection Observer API to observe when the div with ID
-    // back-to-top-buffer has been scrolled entirely out of the viewport,
-    // and adds the btn-back-to-top-visible class to the "Back to top" button
-    // at that point. If the buffer is scrolled back into the viewport, it
-    // removes the class, hiding the button again.
-    var callback = function(entries, observer) {
-        if (entries[0].isIntersecting) {
-            $(".btn-back-to-top").removeClass("btn-back-to-top-visible");
-        } else {
-            $(".btn-back-to-top").addClass("btn-back-to-top-visible");
-        }
-    };
-
-    this.intersectionObserver = new IntersectionObserver(callback);
-    this.intersectionObserver.observe(document.querySelector('#back-to-top-buffer'));
-});

tildes/tildes/templates/base.jinja2

@@ -126,9 +126,6 @@
   </ul>
 </footer>
 
-<div id="back-to-top-buffer" data-js-back-to-top-buffer></div>
-<a href="#" class="btn btn-sm btn-back-to-top">Back to top</a>
-
 {% assets "javascript-third-party" -%}
   <script type="text/javascript" src="{{ ASSET_URL }}"></script>
 {% endassets %}