remove window-border-color variable

packages/rocketchat-theme/server/colors.less

@@ -15,7 +15,6 @@
 @error-contrast: contrast(@error-color);
 @pending-background: lighten(@pending-color, 45%);
 @pending-border: lighten(@pending-color, 30%);
-@window-border-color: @component-color;
 @code-background-color: #f8f8f8;
 @code-border-color: #cccccc;
 @code-font-color: #333333;