remove submodule directory

[j-mew-s]

This reference is hanging around after removing the submodules file and is causing us errors during build:
No url found for submodule path 'cmake/tl-cmake' in .gitmodules

You can also see this error in your Actions run in the Post Run Actions/checkout@v3 step following the commit removing the submodules file e.g. here https://github.com/TartanLlama/expected/actions/runs/4260972315/jobs/7414746037

/usr/bin/git submodule foreach --recursive sh -c "git config --local --name-only --get-regexp 'core\.sshCommand' && git config --local --unset-all 'core.sshCommand' || :"
fatal: No url found for submodule path 'cmake/tl-cmake' in .gitmodules
Warning: The process '/usr/bin/git' failed with exit code 128

Since it was after the run steps, I guess the build counted as successful though.

Disclaimer: I'm new to git submodules, apologies if I've misunderstood something!

[j-mew-s]

Thanks for dealing with this quickly, much appreciated 🙏