Thanks in advance!
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You wont be able to remove just one person from the multi value people or group. What you can do is first get the users from the fields, then remove the desired user.. and then update the field with the remaining users.
To get the users using JSOM, refer the below link: https://pholpar.wordpress.com/2012/11/25/how-to-get-the-users-from-a-multivalued-field-using-jscom/