LTI modedit.php should not display 'disabled' form elements

The LTI form can, depending on how it's been set up, show a whole bunch of disabled elements. 

I can't see any reason for displaying these. If they are not editable then don't display them.

Particular confusion is caused by the 'Select content' button. Even in a disabled state, in most themes users don't realise that this is disabled and leads them to believe that the LTI is broken ("I can't click the content button").