chore(turbo-tasks): Remove old and unused `turbo-tasks-memory` backend (#79560)
I'm deleting all of @sokra's hard work in favor of @sokra's hard work.
RIP memory backend, you served us well 🫡
The new `turbo-tasks-backend` supports both persistent (via LDMB or `turbo-persistence`) and in-memory databases (via the no-op storage implementation), and tends to be faster than `turbo-tasks-memory` when used with the no-op storage. Maintaining both has increasingly become a headache.