mythburn.py: remove the depreciated crop* options to mythffmpeg

The croptop, cropbottom, cropright and cropleft parameters are no longer
supported by mythffmeg so use the new -filter:v "crop=w:h:x:y"
parameter instead.

You would add something like this to your encoding profile

<parameter name="-filter:v"      value="crop=640:480:100:100"/>

I don't have any nuv files so this is completely untested.

Refs #10042.