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

@@ -17,6 +17,11 @@ Versioning].
   * https://github.com/stepancheg/rust-protobuf/issues/551
   * https://github.com/rust-lang/rust/issues/64266
 
+- Remove the temporary workaround for the aforementioned Rust warning, since the
+  newly generated Rust code semi-resolves it. See also:
+
+  * https://github.com/stepancheg/rust-protobuf/issues/551#issuecomment-802221146
+
 ### 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;