Remove check OPF_023

`OPF_023` was checking that there was only one `metadata` element in
the OPF, but:

  * this is already checked by RelaxNG schemas
  * the check is flawed since `collection`s also allow `metadata`
    children

@santoch or @apond please review

(it may be cleaner to first apply #445 and rebase this branch against master afterwards, I can do it if you give the go ahead on the PR).