LibWeb: Make Layout::Node::paint() pure virtual

In the past, the base class implementation of this was used to descend
into subtrees and paint children. That is now taken care of by
StackingContext::paint_descendants() instead, and nothing used this.

Userland/Libraries/LibWeb/Layout/TextNode.cpp

@@ -414,4 +414,8 @@ Optional<TextNode::Chunk> TextNode::ChunkIterator::try_commit_chunk(Utf8View::It
     return {};
 }
 
+void TextNode::paint(PaintContext&, PaintPhase)
+{
+}
+
 }