Remove an out-of-date comment in RegexSet.
Remove an out-of-date comment in RegexSet.

RegexSet uses regex::RegexSet internally since the commit linked
below, which compiles the regexes together.

https://github.com/rust-lang-nursery/rust-bindgen/commit/51f5162f250a5ec10bc5ba8b91b60b45762bc999

emilio/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,rust-lang/rust-bindgen