Remove some obsolete ignored tests

* compile-fail/vec-add.rs is obsolete, there are no mutable
  vectors any more, #2711 is closed
* compile-fail/issue-1451.rs is obsolete, there are no more
  structural records, #1451 is closed
* compile-fail/issue-2074.rs is obsolete, an up to date test
  is in run-pass/nested-enum-same-names.rs, #2074 is closed
* compile-fail/omitted-arg-wrong-types.rs is obsolete, #2093
  is closed
auto merge of #12476 : fhahn/rust/ignored-tests-update2, r=alexcrichton

This PR removes 4 obsolete ignored test cases

* compile-fail/vec-add.rs is obsolete, there are no mutable
  vectors any more, #2711 is closed
* compile-fail/issue-1451.rs is obsolete, there are no more
  structural records, #1451 is closed
* compile-fail/issue-2074.rs is obsolete, an up to date test
  is in run-pass/nested-enum-same-names.rs, #2074 is closed
* compile-fail/omitted-arg-wrong-types.rs is obsolete, #2093
  is closed