Remove some obsolete ignored tests

* compile-fail/vec-add.rs is obsolete, there are no mutable
  vectors any more, rust-lang#2711 is closed
* compile-fail/issue-1451.rs is obsolete, there are no more
  structural records, rust-lang#1451 is closed
* compile-fail/issue-2074.rs is obsolete, an up to date test
  is in run-pass/nested-enum-same-names.rs, rust-lang#2074 is closed
* compile-fail/omitted-arg-wrong-types.rs is obsolete, rust-lang#2093
  is closed

Author: fhahn