Relax checks on unused imports

This is definitely worth a warning but should not make builds fail for
users. In CI builds warnings are denied by default and so this will be
sorted out there before integrating changes.

See issue serialport#169 for more details.

src/lib.rs

@@ -20,7 +20,6 @@
     missing_docs,
     missing_debug_implementations,
     missing_copy_implementations,
-    unused
 )]
 // Document feature-gated elements on docs.rs. See
 // https://doc.rust-lang.org/rustdoc/unstable-features.html?highlight=doc(cfg#doccfg-recording-what-platforms-or-features-are-required-for-code-to-be-present