How to drop orig_to using postfix virtual domains?

With the virtual domains, the final mailbox receiving the email gets the orig_to email address in its header. Is there a way to prevent this (i.e. address translation without keeping the orig_to)?

Maybe you mean X-Original-To header instead orig_to header. String orig_to only appears in maillog. To disable it set parameter enable_original_recipient = no in main.cf

From postconf(5):

enable_original_recipient (default: yes)

Enable support for the X-Original-To message header. This header is needed for multi-recipient mailboxes.

When this parameter is set to yes, the cleanup(8) daemon performs duplicate elimination on distinct pairs of (original recipient, rewritten recipient), and generates non-empty original recipient queue file records.

When this parameter is set to no, the cleanup(8) daemon performs duplicate elimination on the rewritten recipient address only, and generates empty original recipient queue file records.

This feature is available in Postfix 2.1 and later. With Postfix version 2.0, support for the X-Original-To message header is always turned on. Postfix versions before 2.0 have no support for the X-Original-To message header.