Fix indenting of subheadings in TOC

Reduce the specificity of TOC ul styles, so the existing rules
for indenting (with selector `.mw-content-ltr .toc ul ul`) are
applied. This matches the behaviour in other skins.

Bug: T190830
Change-Id: I84fd56c2baeaa218fb4fda493393f8c3d3ddd6cd

resources/screen-common.less

@@ -454,7 +454,6 @@ ul#filetoc {
 	display: block;
 	border: 0;
 }
-#toc ul,
 .toc ul {
 	margin: 1em 0 0;
 	list-style: none;