OK folks.  I just created a script to do it.  I have not used it against a field in a template and would strongly recommend that you never do that.  It does work fine for non-template fields.

Namespace WorkflowActivity.Scripting.SDKScriptRemoveField
    '''<summary>
    '''Provides one or more methods that can be run when the workflow scripting activity is performed.
    '''</summary>
    Public Class UTL_RemoveField
        Inherits RAScriptClass110
        '''<summary>
        '''This method is run when the activity is performed.
        '''</summary>
        Protected Overrides Sub Execute()
            'Write your code here. The BoundEntryInfo property will access the entry, RASession will get the Repository Access session
            Dim inFieldName As String = GetTokenValue("inFieldName")
            Dim BoundFieldValues As FieldValueCollection = BoundEntryinfo.GetFieldValues()
            BoundFieldValues.Remove(inFieldName)
            BoundEntryInfo.SetFieldValues(BoundFieldValues)
            BoundEntryInfo.Save()
        End Sub
    End Class
End Namespace

Why are they using fields that are not assigned to a template?

If you do this using Workflow, there is a checkbox in the Assign Field Values activity to remove fields from unassigned templates.