Auto merge of #4909 - zackmdavis:tidy_paren, r=matklad

unblock rust-lang/rust#46980 by cleaning up an unused pair of parentheses

While this might seem like too trivial of a stylistic nitpick to deserve
its own commit, this [is needed to unblock](https://github.com/rust-lang/rust/pull/46980#issuecomment-355565906) the PR rust-lang/rust#46980 (which makes
the unused-parens lint look at function arguments, which it previously didn't).