Removing attachments is not supported by the core natively.
The main issue is the way that Zammad works with attachments:
Zammad recognizes existing attachments by their checksum and saves affected files once only.
While this saves storage it also means that removing files no longer is trivial.
Imho your “removal via database” potentially leaves references within the system that may give you a hard time in the future. Really, don’t do it.