examples: fix clippy's complaints about redundant_locals

[piodul]

Rust 1.73 got released very recently and introduced a new version of clippy, with new lints. The parallel and parallel-prepared examples contain code which redefines a local variable:

for i in 0..100_000usize {
    // ...
    tokio::task::spawn(async move {
        let i = i;
        // ...
    });
}

This is completely unnecesary and can be removed, as per clippy's suggestion:

error: redundant redefinition of a binding
  --> examples/parallel-prepared.rs:36:9
   |
36 |     for i in 0..100_000usize {
   |         ^
...
44 |             let i = i;
   |             ^^^^^^^^^^
   |
   = help: remove the redefinition of `i`
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_locals
   = note: `#[deny(clippy::redundant_locals)]` on by default

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.