remove redundant cutscene_mark_viewable()

This is already done in `movie::play()` and shouldn't be done again if a script overrides the function.  The extra calls were added in 251285c and cbccf65 (the former copied from the latter), originally as an unsuccessful attempt to fix Mantis 61.

code/mission/missioncampaign.cpp

@@ -1468,7 +1468,6 @@ void mission_campaign_maybe_play_movie(int type)
 		return;
 
 	movie::play(filename);	//Play the movie!
-	cutscene_mark_viewable( filename );
 }
 
 /**

code/missionui/missionscreencommon.cpp

@@ -407,7 +407,6 @@ void common_maybe_play_cutscene(int movie_type, bool restart_music, int music)
 				common_music_close(); 
 				music_off = true;
 				movie::play(The_mission.cutscenes[i].filename);	//Play the movie!
-				cutscene_mark_viewable( The_mission.cutscenes[i].filename );
 			}
 		}
 	}