Revert Subdirectories
This release reverts the change introduced by v2.2.5.1 wherein the nwb schema was stored as a set of submodules. This change was done due to the realization that some MatNWB tooling which do not depend on a valid git installation may break as that is the only way to download a git repository with submodules.
This change also fixes certain tests that may be indicative of changes to MATLAB's 2020b release.