Revert "[modules] Enable cxxmodules for LLVM and libstdc++."
Revert "[modules] Enable cxxmodules for LLVM and libstdc++."

This reverts commit 86c95d966c63a6f702759e1c255ba5415a348aef.

We are not quite ready to do this. Our PR infrastructure gave
green light while it shouldn't have.

olifre/root,karies/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,root-mirror/root,olifre/root,olifre/root,olifre/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,karies/root,olifre/root,root-mirror/root,karies/root,olifre/root,karies/root,karies/root,olifre/root,root-mirror/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,root-mirror/root,root-mirror/root