Auto merge of #7367 - alexcrichton:less-fn-box, r=ehuss
Auto merge of #7367 - alexcrichton:less-fn-box, r=ehuss

Remove another `FnBox` trait

No longer needed on stable!

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