Modify emailed reports content
The email body content starts here for csv: https://github.com/civicrm/civicrm-core/blob/5.18.4/CRM/Report/Form.php#L3413, and a bit lower down for pdf.

Note also the attachments are hardcoded in, a couple lines lower down.

Note that in the report itself on the Title and Format tab you can add some text/html at the end of the header which will get included in the email.

The code seems to be after hook_civicrm_alterReportVar so I don't think you can use that to remove the attachments, but you should be able to use hook_civicrm_alterMailParams to remove the attachments.