gh-100227: Revert gh-102925 "gh-100227: Make the Global Interned Dict Safe for Isolated Interpreters" (gh-103063)

This reverts commit 87be8d9.

This approach to keeping the interned strings safe is turning out to be too complex for my taste (due to obmalloc isolation). For now I'm going with the simpler solution, making the dict per-interpreter. We can revisit that later if we want a sharing solution.