Remove test case that assumes FromStr provides TryFrom<&'a str>.
Remove test case that assumes FromStr provides TryFrom<&'a str>.

See https://travis-ci.org/rust-lang/rust/jobs/269861252

aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust