Remove scrollbar customizations (#29800)

Fixes go-gitea/gitea#29652. Removes all
scrollbar customization as per popular vote on
go-gitea/gitea#29652 (comment).

There is one more case of `-webkit-scrollbar` left in CSS and
go-gitea/gitea#29400 will get rid of that as
well.

(cherry picked from commit 0827552d9ab6bec5fccef86139cbad3ae7b582b7)

Author: silverwind

web_src/css/base.css

@@ -116,33 +116,9 @@ h1.error-code {
 }
 
 * {
-  scrollbar-color: var(--color-primary) transparent;
   caret-color: var(--color-caret);
 }
 
-::-webkit-scrollbar {
-  width: 10px;
-  height: 10px;
-}
-
-::-webkit-scrollbar-thumb {
-  box-shadow: inset 0 0 0 6px var(--color-primary);
-  border: 2px solid transparent;
-  border-radius: var(--border-radius);
-}
-
-::-webkit-scrollbar-thumb:window-inactive {
-  box-shadow: inset 0 0 0 6px var(--color-primary);
-}
-
-::-webkit-scrollbar-thumb:hover {
-  box-shadow: inset 0 0 0 6px var(--color-primary-dark-2);
-}
-
-::-webkit-scrollbar-corner {
-  background: transparent;
-}
-
 ::file-selector-button {
   border: 1px solid var(--color-light-border);
   color: var(--color-text-light);