Remove the variable islandora_video_retain_original because it is never used

The first option in the Video SP's admin form is "Keep original file after ingest", which sets the variable `islandora_video_retain_original`. This variable is never called.

This variable was brought in as `islandora_video_sp_retain_original` in the original port to D7 (ported from `video_retain_original`). That commit also removes the code that actually acted on this variable, at the line linked below.

https://github.com/Islandora/islandora_solution_pack_video/commit/56ac3fc73df462922dcbc0de595ad996d6dcb277#diff-6379deff5ae65093d6a752e1c09937c5L104