Turn off hard errors for lints (#3441)

It happened multiple times now that I wanted to quickly execute a test, but because of a warning that had become an error, it didn't execute.

This turns warnings into warnings again; our CI will fail if there is a warning, anyway (because of RUSTFLAGS: -Dwarnings)

deltachat-ffi/src/lib.rs

@@ -1,4 +1,4 @@
-#![deny(unused, clippy::all)]
+#![warn(unused, clippy::all)]
 #![allow(
     non_camel_case_types,
     non_snake_case,

src/lib.rs

@@ -2,7 +2,7 @@
 
 #![recursion_limit = "256"]
 #![forbid(unsafe_code)]
-#![deny(
+#![warn(
     unused,
     clippy::correctness,
     missing_debug_implementations,