Remove the WebKit-specific scrollbar styling altogether

It is preferable to remove WebKit-specific styling and use the browser
and OS default scrollbars.
Thanks to comments from @julianfortune, @arniu, and @ehuss.

Closes #1483.

Author: noritada

src/theme/css/chrome.css

@@ -2,14 +2,6 @@
 
 @import 'variables.css';
 
-::-webkit-scrollbar {
-    background: var(--bg);
-    width: 8px;
-}
-::-webkit-scrollbar-thumb {
-    background: var(--scrollbar);
-    border-radius: 4px;
-}
 html {
     scrollbar-color: var(--scrollbar) var(--bg);
 }