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

@@ -3924,8 +3924,6 @@ raw_expression_tree_walker(Node *node,
 
 				if (walker(coldef->typeName, context))
 					return true;
-				if (walker(coldef->compression, context))
-					return true;
 				if (walker(coldef->raw_default, context))
 					return true;
 				if (walker(coldef->collClause, context))