chore: remove no longer needed features

examples/repl/main.rs

@@ -3,7 +3,6 @@
 //!
 //! Usage:  cargo run --example repl --release -- <databasefile>
 //! All further options can be set using the set-command (type ? for help).
-#![feature(ptr_cast)]
 
 #[macro_use]
 extern crate deltachat;

src/lib.rs

@@ -3,7 +3,7 @@
 #![warn(clippy::all, clippy::perf, clippy::not_unsafe_ptr_arg_deref)]
 // This is nice, but for now just annoying.
 #![allow(clippy::unreadable_literal)]
-#![feature(c_variadic, ptr_wrapping_offset_from, ptr_cast)]
+#![feature(ptr_wrapping_offset_from)]
 
 #[macro_use]
 extern crate failure_derive;