refactor(turbo-tasks-backend): Remove `ExecuteContextImpl::lower_read_transaction` (#80822)
Following the removal of the other `transaction` and `lower_read_transaction` callsites in #80816...
If `ExecuteContextImpl::transaction` is inlined into `ExecuteContextImpl::restore_task_data` (the only remaining callsite), we no longer need `lower_read_transaction` and its crimes against Rust lifetimes.
Sanity checked with:
```
cargo check --features turbo-tasks-backend/lmdb
```