remove residual code from PR #320, which is no longer useful

[lsylvestre]

PR #481 allows external pre-compiled code to be used in the grading environment, effectively replacing the existing mechanism for test code reuse (PR #320).

This existing mechanism is no longer supported (it is now ignored by grader/grading.ml), but residual code continues to run. In particular, an exception is raised when file depend.txt is present in the exercise folder and specifies invalid file paths.

This new PR removes residual code from PR #320

This closes issue #366