Everywhere: Stop using NonnullRefPtrVector

This class had slightly confusing semantics and the added weirdness
doesn't seem worth it just so we can say "." instead of "->" when
iterating over a vector of NNRPs.

This patch replaces NonnullRefPtrVector<T> with Vector<NNRP<T>>.

Userland/Libraries/LibWeb/CSS/StyleComputer.h

@@ -43,7 +43,7 @@ private:
     explicit PropertyDependencyNode(String name);
 
     String m_name;
-    NonnullRefPtrVector<PropertyDependencyNode> m_children;
+    Vector<NonnullRefPtr<PropertyDependencyNode>> m_children;
     bool m_marked { false };
 };