Remove temporary workaround for rust-protobuf

Remove #![allow(soft_unstable)], which was previously added to
workaround an issue in rust-protobuf [1]. This issue has been
semi-resolved upstream [2], so we can deny this warning once more.

[1]: stepancheg/rust-protobuf#551
[2]: stepancheg/rust-protobuf#551 (comment)

Author: apyrgio

CHANGELOG.md

@@ -7,16 +7,6 @@ Versioning].
 
 ## [Unreleased]
 
-### Fixed
-
-- Temporarily fix a build error for nightly Rust. In a nutshell, the generated
-  Rust code for our proto files triggers a compiler warning in nightly Rust,
-  which we ultimately treat as an error. Until this is fixed upstream, we
-  silence this warning. See also:
-
-  * https://github.com/stepancheg/rust-protobuf/issues/551
-  * https://github.com/rust-lang/rust/issues/64266
-
 ### Changed
 
 - Bump the dialoguer dependency to v0.8.0, to fix a compilation error.

src/lib.rs

@@ -39,9 +39,6 @@
     unused_results,
     variant_size_differences
 )]
-// FIXME: This must be removed once this is solved:
-// https://github.com/stepancheg/rust-protobuf/issues/551
-#![allow(soft_unstable)]
 
 pub mod aead;
 pub mod cryptors;