Remove another `FnBox` trait
Remove another `FnBox` trait

No longer needed on stable!

rust-lang/cargo,eminence/cargo,nipunn1313/cargo,tshepang/cargo,vojtechkral/cargo,nipunn1313/cargo,tshepang/cargo,eminence/cargo,rust-lang/cargo,tsurai/cargo,tsurai/cargo,tshepang/cargo,rustyhorde/cargo,vojtechkral/cargo,rust-lang/cargo,vojtechkral/cargo,eminence/cargo,tsurai/cargo,eminence/cargo,rust-lang/cargo,tsurai/cargo,rustyhorde/cargo,rustyhorde/cargo,vojtechkral/cargo,rustyhorde/cargo,tsurai/cargo,nipunn1313/cargo,tshepang/cargo,nipunn1313/cargo