[Grid] Move header rule into search rules

This will also apply to the grid layout so this seems a more
appropriate place, given the class relates to search.

Bug: T303484
Change-Id: Ib5b57fd948ecb5eefbf1d66e0e71045aed6e3102

resources/skins.vector.styles/components/Header.less

@@ -37,10 +37,6 @@
 	.vector-search-box {
 		float: left;
 		z-index: @z-index-menu;
-
-		.vector-layout-legacy & {
-			flex-grow: 1;
-		}
 	}
 
 	&-content,

resources/skins.vector.styles/components/VueEnhancedSearchBox.less

@@ -24,6 +24,8 @@
 	// Support IE 9-11, Trident cuts values 2 digits after decimal point.
 	// `calc` enables to set correct calculation in place again. See T102364.
 	font-size: @font-size-base--trident-hack;
+	// If this is a flex item, make sure it grows into available space.
+	flex-grow: 1;
 }
 
 // Typeahead search elements