When creating an invitation, a calendar attachment is included with the message by default.
There are two places where you can choose to turn this option off. The first will be when you are designing your email. In the Advanced Options section on the right, unselect the Attach Calendar Invite checkbox.
The second place will be in the Review step of creating your email. If you scroll down you will see a Calendar Invite Row where you can click Remove to get rid of the invite.