Remove nightly Rust clippy test

[mgeisler]

Description of changes:

All PRs currently fail due to a Clippy lint:

warning: assigning the result of `Clone::clone()` may be inefficient
   --> mls-rs/src/tree_kem/tree_hash.rs:193:13
    |
193 |             filtered_sets[n] = filtered_sets[p as usize].clone();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `clone_from()`: `filtered_sets[n].clone_from(&filtered_sets[p as usize])`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#assigning_clones
    = note: `#[warn(clippy::assigning_clones)]` on by default

Trying to fix this as recommended results in a compilation error since we cannot call a mutating method on filtered_sets while we also read from it…

In general, Nightly Rust is a moving target (moving faster much than stable) and as such, it’s inconvenient to have Clippy fail builds due to new (experimental) lints being enabled there.

There are already two Clippy runs (using stable Rust) in the native_build.yml file, so I simply removed the unstable Rust run from no_std__build.yml.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT license.

[mulmarta]

I fully agree that running nightly causes problems -- but why remove clippy altogether? We can run stable clippy

[tomleavy]

Closing since #108 covered this

[mgeisler]

I fully agree that running nightly causes problems -- but why remove clippy altogether? We can run stable clippy

It looked like the Clippy checks here were redundant with the ones in the normal build (I might be wrong, but I don't think a no_std build will trigger other lint warnings than a regular build).

But I'm glad it's fixed now!