Remove the second lifetime from DiagnosticArg.

Because it's always static.

I'm surprised the compiler allowed this unused lifetime without any
complaint.

compiler/rustc_errors/src/translation.rs

@@ -13,7 +13,7 @@ use std::error::Report;
 /// Typically performed once for each diagnostic at the start of `emit_diagnostic` and then
 /// passed around as a reference thereafter.
 pub fn to_fluent_args<'iter>(
-    iter: impl Iterator<Item = DiagnosticArg<'iter, 'static>>,
+    iter: impl Iterator<Item = DiagnosticArg<'iter>>,
 ) -> FluentArgs<'static> {
     let mut args = if let Some(size) = iter.size_hint().1 {
         FluentArgs::with_capacity(size)