Removing the select all option in a custom field

Hello, hopefully this is an easy question to solve. 

I need to add a field on a custom form using the drop down option.  We want the user to be able to select more than one option, but I don't want the option to select "all" to appear.  It seems like the "select all" option is baked into the field.  Is there anyway to remove it?  Or does anyone have a suggested workaround? 

Thank you. 

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