rdiff_backup.connection: Small refactor in VirtualFile

rdiff_backup/connection.py

@@ -552,9 +552,8 @@ class VirtualFile:
 
 	@classmethod
 	def closebyid(cls, id):
-		fp = cls.vfiles[id]
+		fp = cls.vfiles.pop(id)
 		cls.id_generator.remove(id)
-		del cls.vfiles[id]
 		return fp.close()
 
 	@classmethod