Remove simple_sequence

[tcbrindle]

This was added very early in the life of Flux with the intention of making it easy to write Flow-style (or Rust-style) iterators with just a next() -> optional method.

It works fine, but it's not particularly useful -- if you're doing a quick prototype then flux::generator is quicker and easier, and otherwise a "full" sequence implementation isn't much more work and usually gets you multipass for free.

So given that it's not useful and it's not used anywhere, let's get rid of it.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.30%. Comparing base (0ebf699) to head (14effc2). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #200 +/- ## ======================================= Coverage 98.30% 98.30% ======================================= Files 70 70 Lines 2471 2471 ======================================= Hits 2429 2429 Misses 42 42 ```

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