Hardcoded number of viewlets in CSS (fixes #15969)

src/vs/workbench/parts/explorers/media/treeExplorer.contribution.css

@@ -12,12 +12,6 @@
 	background: url('Refresh_inverse.svg') center center no-repeat;
 }
 
-/* Coerce extension-contributed viewlet icon into a style similar to stock icons */
-.monaco-workbench > .activitybar .monaco-action-bar .action-item:nth-child(n+6) .action-label {
-	-webkit-filter: grayscale(1) invert(1);
-	filter: grayscale(1) invert(1);
-}
-
 .custom-viewlet-tree-node-item {
 	height: 22px;
 	line-height: 22px;