ml-dsa: remove std feature (#893)

It doesn't appear this feature actually does anything except
transitively activate the `sha3/std` feature, which we likely plan on
removing in the next breaking release.

Now that `core::error::Error` is stable, we are generally trying to
remove `std` features across the @RustCrypto project, except in cases
where libraries do things like e.g. file I/O (for loading/saving keys)
and actually have a legitimate `std` dependency that way.

ml-dsa/Cargo.toml

@@ -15,8 +15,7 @@ categories = ["cryptography"]
 keywords = ["crypto", "signature"]
 
 [features]
-default = ["std", "rand_core"]
-std = ["sha3/std"]
+default = ["rand_core"]
 zeroize = ["dep:zeroize", "hybrid-array/zeroize"]
 rand_core = ["dep:rand_core", "signature/rand_core"]