Fix submodule problem

[RagnarGrootKoerkamp]

Currently the BAPCtools repo must be a git submodule of the contest repo in order to make the validation.h symlinks work. It would be nice if we could circumvent this requirement.

Possible solutions:

• Make each validation.h a copy. This is problematic if bugs are found.
• Remove the symlink, and add the headers directory to the paths the compiler searches for header files. Small drawback is that it won't be possible to compile the IO validators without the BAPCtools, and the validators will need to be included when exporting to DJ/kattis.

[RagnarGrootKoerkamp]

Fixed by 1bafccf.