It seems that blocks are written to disk in the WriteBlockToDisk function in main.cpp. But I haven't found where they are deleted. Maybe they don't ever get deleted from disk? Once written they are always valid?
They will never ever delete themself and you should never delete them, deleting any part of the blockchain would invalidate it and will require you to download them again. Unless they get corrupted, they will be valid. The Bitcoin core verify every single of the block during start up and they will inform you if it gets corrupted.