[O] org-agenda-highlight-todo may erroneously pickup icon 'display property.

[Keith David Bershatsky]

Depending upon a user's `org-agenda-prefix-format` (e.g., "%i %-10:c% t%s"), 
`org-agenda-highlight-todo` may erroneously pickup an icon `'display` property 
and then include/duplicate that icon in a new space between the todo-keyword 
and the priority.  I chose tho fix this in my own setup with `org-plist-delete` 
to remove the `'display` pair of PROPERTY / VALUE from the plist of text 
properties that are copied to the new space " " that is concatenated between 
the todo-keyword and the priority.

Thanks,

Keith