rectangles: rm unneeded `return`
rectangles: rm unneeded `return`

Clippy suggested:
https://travis-ci.org/exercism/rust/builds/456356560
warning: unneeded return statement
    = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#needless_return

petertseng/exercism-rust,petertseng/exercism-rust,exercism/xrust,petertseng/exercism-rust,exercism/xrust