Downgrade trivial_regex to nursery

[dtolnay]

See #6690. I think there is still value in a trivial_regex lint, but only if clippy can tell that the regex is only ever constructed and applied to a single input.

let regex = Regex::new("trivial_regex")?;
println!("{}", regex.is_match(s));
// `regex` never used again

---

changelog: remove trivial_regex from default set of enabled lints

[rust-highfive]

r? @Manishearth

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[Manishearth]

@bors r+

[bors]

📌 Commit fd35517 has been approved by Manishearth

[bors]

⌛ Testing commit fd35517 with merge 4bbd7e4...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Manishearth
Pushing 4bbd7e4 to master...