Stop retaining output files from past submission in BatchAndOutput

[birka0]

In BatchAndOutput tasks, some test cases may be "output-optional": the user can provide an output file for these cases, but if they don't, the compiled program will be used for evaluation instead.

However, with the current mechanics of the CMS, if a contestant submits an output file for an output-optional testcase even once, that file is retained for all subsequent submissions. This makes it impossible to execute the contestant’s program for that test case in any later submission!

I believe the intended (and far more useful) behavior would be to always execute the program when an output file is not included in the current submission.