fix(rustup): type-inference fails on empty vec

Previously this wasn't the case, as the type could be inferred by the
type of the parent-vector to extend.

Apparently this feature was removed, probably for good reason.

gen/admin2_email_migration-cli/src/main.rs

@@ -104,7 +104,6 @@ impl<'n, 'a> Engine<'n, 'a> {
                         err.issues.push(CLIError::UnknownParameter(key.to_string(), 
                                                                   {let mut v = Vec::new();
                                                                            v.extend(self.gp.iter().map(|v|*v));
-                                                                           v.extend([].iter().map(|v|*v));
                                                                            v } ));
                     }
                 }