[gnome-shell] theme: Remove invalid button styling

[Jakub Steiner <jimmac src gnome org>]

commit 9719b13cf855657909377f7257dae02dbbe04cfc
Author: nana-4 <hnmaigo gmail com>
Date:   Sun Jan 12 16:22:59 2020 +0900

    theme: Remove invalid button styling
    
    There is no ".message-list-clear-button .button" selector, but
    ".message-list-clear-button.button". To make it clear, this commit adds
    .button to .message-list-clear-button.
    
    https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/931

 data/theme/gnome-shell-sass/widgets/_message-list.scss | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)
---
diff --git a/data/theme/gnome-shell-sass/widgets/_message-list.scss 
b/data/theme/gnome-shell-sass/widgets/_message-list.scss
index b0e2b491dc..0a68c90b9f 100644
--- a/data/theme/gnome-shell-sass/widgets/_message-list.scss
+++ b/data/theme/gnome-shell-sass/widgets/_message-list.scss
@@ -23,11 +23,8 @@
 }
 
 // clear button
-.message-list-clear-button {
+.message-list-clear-button.button {
   margin:$base_margin $base_margin*2;
-  .button {
-    @extend %button;
-  }
 }
 
 // message bubbles