Remove inappropriate raw_expression_tree_walker() code

It was walking into the ColumnDef->compression field, which is not a
node but a string.  This code is currently not reachable (because the
compression field is only set in situations that don't go through
raw_expression_tree_walker()), but if it had been, this could have
behaved erratically.

src/backend/nodes/nodeFuncs.c

@@ -4262,8 +4262,6 @@ raw_expression_tree_walker_impl(Node *node,
 
 				if (WALK(coldef->typeName))
 					return true;
-				if (WALK(coldef->compression))
-					return true;
 				if (WALK(coldef->raw_default))
 					return true;
 				if (WALK(coldef->collClause))