I know there is a add attachment like
Do we have a remove attachment command such as
No. There doesn't seem to be any native facility for this although I think it would be relatively straightforward to add. Currently, however, you must discard the entire email.
Note also the following from the iPhone OS Reference Library:
Important: The mail composition interface itself is not customizable and must not be modified by your application. In addition, after presenting the interface, your application is not allowed to make further changes to the email content. The user may still edit the content using the interface, but programmatic changes are ignored. Thus, you must set the values of content fields before presenting the interface.
Ok, what I've done is to discard the entire email and re-attach the attachment.