Remove no longer needed special handling for Firefox

The highlighting was removed in Firefox when the cursor was no longer
moving to handle the behaviour of reporting a file drag and then
providing no files in the drop event. That behaviour (which was only
present in Firefox 48 and 49) is already handled with the "dropnofiles"
callback, so that special handling is no longer needed.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>