Revert "Test that static_assert actually works by using it in main.cpp."

This reverts commit 8d9c058118361e39831bbc5adc47e6f6c6afd5df.
Revert "To test that nullptr actually works, use it in one file."

This reverts commit 4a86c4367e9212ba18cfab48e051c2b68491bec2.