Remove 'overflow: auto' from .box() mixin

I am pretty sure this is unnecessary in almost all places this mixin
is used, and at least one of them (for 'fieldset') is causing issues
(T181258).

The only place I am not pretty sure about is in the .nav-block()
mixin, so let's add it back there.

Bug: T181258
Change-Id: I77d34634d97b38712e74bc5f2227e42e9c49f18d

resources/variables.less

@@ -93,7 +93,6 @@
 	border-width: 1px 1px @border;
 	padding: 1.25em 1.75em;
 	box-shadow: 0 1px 2px 1px rgba( 0, 0, 0, 0.02 );
-	overflow: auto;
 }
 
 // Icons for the page actions menus
@@ -110,6 +109,7 @@
 
 .nav-block() {
 	.box();
+	overflow: auto;
 	padding: 1.5em 1.5em 0;
 	margin: 1em 0;
 	line-height: 1.1;