OOUI: Remove confusing 'position' override

Vector is the only skin that does this, setting 'position: relative'
for .oo-ui-windowManager-modal (normally 'position: static') and for
.ve-ui-overlay-global (normally 'position: absolute').

The override for .oo-ui-windowManager-modal caused it to create a new
stacking context, which is the only reason the 'z-index' override
worked. Use the right selector to override 'z-index' instead.

The override for .ve-ui-overlay-global was completely pointless and
it's surprising that it never broke anything.

Change-Id: Icd1dec43e2da9ef2090b18145099838de3a7890a

skinStyles/ooui.less

@@ -1,9 +1,8 @@
 @import '../variables.less';
 
-.oo-ui-windowManager-modal,
+.skin-vector .oo-ui-windowManager-modal > .oo-ui-dialog,
 .skin-vector .ve-ui-overlay-global {
 	z-index: 101;
-	position: relative;
 }
 
 .oo-ui-defaultOverlay {