Remove the UntranslatableDiagnosticTrivial lint.

It's a specialized form of the `UntranslatableDiagnostic` lint that is
deny-by-default.

Now that `UntranslatableDiagnostic` has been changed from
allow-by-default to deny-by-default, the trivial variant is no longer
needed.

clippy_config/src/lib.rs

@@ -6,7 +6,6 @@
     clippy::missing_panics_doc,
     rustc::diagnostic_outside_of_impl,
     rustc::untranslatable_diagnostic,
-    rustc::untranslatable_diagnostic_trivial
 )]
 
 extern crate rustc_ast;