style.css: Remove custom .devel style class

GTK includes it's own .devel style class, so we don't need to
override by creating our own.

podcasts-gtk/resources/gtk/style.css

@@ -18,15 +18,3 @@ list, border {
 .player-show-label {
     font-size: smaller;
 }
-
-.devel headerbar {
-  background: transparent -gtk-icontheme("system-run-symbolic") 80% 0/128px 128px no-repeat,
-              linear-gradient(to left,
-                              mix(@theme_selected_bg_color, @theme_bg_color, 0.5) 8%,
-                              @theme_bg_color 25%);
-  color: alpha(@theme_fg_color, 0.1);
-}
-
-.devel headerbar label {
-  color: @theme_fg_color;
-}